diff --git a/server_environment/models/server_env_mixin.py b/server_environment/models/server_env_mixin.py
index e45052829..3c8c32ea9 100644
--- a/server_environment/models/server_env_mixin.py
+++ b/server_environment/models/server_env_mixin.py
@@ -439,6 +439,13 @@ def _preserve_not_env_managed_data(self, field_name_list):
 
         This method forces to 'persist' these values if they are not
         explicitly overridden by the current environment configuration.
+
+        Note: if a field is already server-env managed (ie it already has a
+        value stored in its ``<field>_env_default`` companion field, for
+        instance because the mixin was already applied to it by another
+        module before), its raw column may only contain stale data from
+        before it became a non-stored field. In that case, we must not
+        overwrite the existing (up to date) default value with it.
         """
         self.env.cr.execute(f"SELECT * FROM {self._table}")
         for row in self.env.cr.dictfetchall():
@@ -450,6 +457,15 @@ def _preserve_not_env_managed_data(self, field_name_list):
             if record:
                 record_values = {}
                 for field_name in field_name_list:
-                    if field_name in row:
-                        record_values[field_name] = row[field_name]
-                record.update(record_values)
+                    if field_name not in row:
+                        continue
+                    default_field = self._server_env_default_fieldname(field_name)
+                    if default_field and record[default_field]:
+                        # A value is already preserved for this field
+                        # (eg. set by a previous server-env managed version
+                        # of the field): keep it, do not clobber it with the
+                        # (possibly stale) raw column value.
+                        continue
+                    record_values[field_name] = row[field_name]
+                if record_values:
+                    record.update(record_values)
diff --git a/server_environment/tests/__init__.py b/server_environment/tests/__init__.py
index 73093474f..6c0446da9 100644
--- a/server_environment/tests/__init__.py
+++ b/server_environment/tests/__init__.py
@@ -1,2 +1,3 @@
 from . import test_server_environment
 from . import test_environment_variable
+from . import test_preserve_not_env_managed_data
diff --git a/server_environment/tests/fake_models.py b/server_environment/tests/fake_models.py
new file mode 100644
index 000000000..b2a76f2ba
--- /dev/null
+++ b/server_environment/tests/fake_models.py
@@ -0,0 +1,22 @@
+# License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
+from odoo import api, models
+
+# pylint: disable=consider-merging-classes-inherited
+
+
+class FakePartner(models.Model):
+    _name = "res.partner"
+    _inherit = ["res.partner", "server.env.mixin"]
+
+    @property
+    def _server_env_fields(self):
+        base_fields = super()._server_env_fields
+        partner_fields = {
+            "city": {},
+        }
+        partner_fields.update(base_fields)
+        return partner_fields
+
+    @api.model
+    def _server_env_global_section_name(self):
+        return "partner"
diff --git a/server_environment/tests/test_preserve_not_env_managed_data.py b/server_environment/tests/test_preserve_not_env_managed_data.py
new file mode 100644
index 000000000..d7579f9b7
--- /dev/null
+++ b/server_environment/tests/test_preserve_not_env_managed_data.py
@@ -0,0 +1,102 @@
+# Copyright 2026 Camptocamp SA
+# License LGPL-3.0 or later (https://www.gnu.org/licenses/lgpl.html)
+
+from odoo_test_helper import FakeModelLoader
+
+from . import common
+
+
+class TestPreserveNotEnvManagedData(common.ServerEnvironmentCase):
+    @classmethod
+    def setUpClass(cls):
+        super().setUpClass()
+        cls.loader = FakeModelLoader(cls.env, cls.__module__)
+        cls.loader.backup_registry()
+        cls._origin_fields = {}
+        for model in ("res.partner", "res.users"):
+            cls._origin_fields[model] = set(dir(cls.env[model].__class__))
+
+    def remove_mixin_fields(self):
+        """Clean up what FakeModelLoader.restore_registry() leaves behind.
+
+        It never touches non-field attributes (e.g. _inverse_server_env_city).
+        """
+        for model in ("res.partner", "res.users"):
+            extra = set(self.env[model].__class__.__dict__) - self._origin_fields[model]
+            for attr in extra:
+                delattr(self.env[model].__class__, attr)
+
+    def setUp(self):
+        super().setUp()
+        from .fake_models import FakePartner
+
+        self.loader.update_registry((FakePartner,))
+        self.addCleanup(self.loader.restore_registry)
+        self.addCleanup(self.remove_mixin_fields)
+        # Deliberately not setting "city" here: writing it would already
+        # populate city_env_default through the mixin's inverse method,
+        # which would defeat the "no default yet" scenario below.
+        self.partner = self.env["res.partner"].create({"name": "Test partner"})
+
+    def _set_raw_city_column(self, value):
+        """Bypass the ORM to simulate a stale/orphaned raw column value.
+
+        Once a field is taken over by the mixin it becomes non-stored, so
+        the ORM never reads or writes its physical column again. The column
+        itself is never dropped though, so it can keep holding old data from
+        before the field became server-env managed.
+        """
+        self.env.cr.execute(
+            "UPDATE res_partner SET city = %s WHERE id = %s",
+            (value, self.partner.id),
+        )
+        self.partner.invalidate_recordset(["city"])
+
+    def test_preserve_rescues_value_when_no_default_yet(self):
+        """Rescue the raw column value when there is no default yet.
+
+        First-time adoption: no default stored yet, so the raw column
+        value must be rescued into the new default field.
+        """
+        self._set_raw_city_column("Legacy Raw City")
+        self.env["res.partner"]._preserve_not_env_managed_data(["city"])
+        self.partner.invalidate_recordset()
+        self.assertEqual(self.partner.city_env_default, "Legacy Raw City")
+        self.assertEqual(self.partner.city, "Legacy Raw City")
+
+    def test_preserve_does_not_overwrite_existing_default(self):
+        """Do not overwrite a default value that is already set.
+
+        A default already set (e.g. because the field was already
+        server-env managed by another module before) must not be clobbered
+        by a stale raw column value.
+        """
+        # Simulate the field having already been server-env managed: a
+        # legitimate, up to date default is already stored.
+        self.partner.write({"city": "Current Default City"})
+        self.assertEqual(self.partner.city_env_default, "Current Default City")
+        # The underlying (now unused) raw column still holds ancient data
+        # from before the field became non-stored.
+        self._set_raw_city_column("Ancient Stale City")
+        self.env["res.partner"]._preserve_not_env_managed_data(["city"])
+        self.partner.invalidate_recordset()
+        self.assertEqual(self.partner.city_env_default, "Current Default City")
+        self.assertEqual(self.partner.city, "Current Default City")
+
+    def test_preserve_ignores_unknown_column(self):
+        """Fields without a matching raw column are silently skipped."""
+        # Must not raise even though the field name doesn't exist as a
+        # column on the table.
+        self.env["res.partner"]._preserve_not_env_managed_data(["field_not_a_column"])
+
+    def test_preserve_does_not_leak_into_env_configured_field(self):
+        """Keep reading from the environment when a config key is defined.
+
+        When a config key is defined, the field must keep reading from
+        the environment, regardless of any raw column value.
+        """
+        self._set_raw_city_column("Legacy Raw City")
+        with self.load_config(public="[partner]\ncity = From Env\n"):
+            self.env["res.partner"]._preserve_not_env_managed_data(["city"])
+            self.partner.invalidate_recordset()
+            self.assertEqual(self.partner.city, "From Env")
diff --git a/test-requirements.txt b/test-requirements.txt
new file mode 100644
index 000000000..4ad8e0ece
--- /dev/null
+++ b/test-requirements.txt
@@ -0,0 +1 @@
+odoo-test-helper