Offer Add-ons on Products no longer save in latest Ultimate Multisite

[adoreyourstore]

Description:
I’m upgrading from WP Ultimo 2.3.3 where “Offer Add-ons” on Products (Plans) works correctly.

In 2.4.x (Ultimate Multisite), I can select a Service in the Offer Add-ons field on a Product and it shows as selected. After I click Save Product, the page reloads and the field is empty again.

Steps to reproduce:

Network install Ultimate Multisite 2.4.x on a fresh multisite.

Create a Service product.

Create a Plan product.

In the Plan, go to Up & Downgrades → Offer Add-ons and select the Service.

Click Save Product.

Reload the page.

Expected:
The selected Service remains saved in the Offer Add-ons field.

Actual:
The field is empty; no Offer Add-ons are saved.

Environment:

WP Ultimo / Ultimate Multisite: 2.4.x

WordPress: 6.8.3

PHP: 8.3.25

Tested on a clean network with only Ultimate Multisite active.

Same setup on WP Ultimo 2.3.3 works correctly (Offer Add-ons persist).

This appears to be a regression introduced after 2.3.3.