I don’t know the details, but I do know that the midi support changes the definition of another baud rate like 38,000-Something to come out as 31,250, and they change the clock of the uart in order to make that happen. I’m sorry to make it sound vague and mysterious, but that’s my level of understanding.
I think if you search this forum with some of those search terms you can find the details. And reading the hardware schematics.
edit: here’s the link to the config.txt change needed - it’s a long post, but points to some doc;
And the relevant section is:
Adjust Serial Port Clock to allow MIDI baudrate 31250
[pi2]
init_uart_clock=2441000
init_uart_baud=38400
dtparam=uart0_clkrate=3000000
[pi3]
init_uart_clock=39062500
init_uart_baud=38400
dtparam=uart0_clkrate=48000000
[all]