Question about Kakute H7 V2's UART baudrate

I am trying to connect Kakute H7 v2 and my SBC via uart.
After several tests, I found that the baudrate of Kakute h7 v2 serial3 is 460800 as the maximum value.
If I have entered a baudrate (serial3_baud) higher than 460800, could not able to connect to the SBC’s UART via mavproxy.

$ mavproxy.py --master=/dev/ttyTHS1 --baudrate=500000
Connect /dev/ttyTHS1 source_system=255
Log Directory:
Telemetry log: mav.tlog
Waiting for heartbeat from /dev/ttyTHS1
MAV> link 1 down

$ mavproxy.py --master=/dev/ttyTHS1 --baudrate=460800
Connect /dev/ttyTHS1 source_system=255
Log Directory:
Telemetry log: mav.tlog
Waiting for heartbeat from /dev/ttyTHS1
MAV> Detected vehicle 1:1 on link 0
online system 1
STABILIZE> Mode STABILIZE

I would like to know Baudrate=460800 is the maximum baudrate of Kakute H7 V2 or it has any other hiden parameters.

As a result of further testing, my SBC and the UART of Kakute H7 v2 are connected but not communicating.
I tried changing various parameters but to no avail and I don’t know why.