ModbusRTUServer not working on baudrate above 32000 in Arduino mega 2560

[dali0]

I'm using an Arduino mega 2560 as a slave on rs485 networks where the master is pc with a python script on baudrate 115200
the ArduinoModbus is working fine with baudrate under 32000 no problem.
but it's freezing on the poll() function on any baud rate above 32000
ModbusRTUServer.poll();
get stuck a few ms and then continue the loop but with no reply or error
note : i have tested the same setup on baudrate 115200 in stm32F446RE and its working fine!