RaspberryPi 4 - high baud rate receive data continuously

[TopgunZh]

I modify the serial baud 2M，and i test it.The sending data is work well,but the reception function is a problem.It only one character can be accepted at a time.
For example, i sned 0xAA 0xBB 0xCC at a time,it just receive 0xAA.Then i send again,i get the same result,just receive one character at a time.
The receive and send function work well in 9600,115200 but not in 1M,2M.

Please help me,thanks.