I share m5 support answer on the topic
"UART0 is a specifically assigned for log print, it is encapsulated by ESP32 at lower layer. It will print the system log automatically and seems like we users are not permitted to shut it down, so we don’t recommend users to use UART0 as a normal UART series port, it will interrupt you communication. That why we connect it with CP2104."
to me it's clear the usage of UART0 ..less cleare why this two GPIO are routed to the M5bus since can't be used
I will use UART2 for my custom project with this assignment GPIO34=RX GPIO17=TX