Well, my use case is quite different.
The receiver (1936) which runs MIBAM, does nothing besides BAMing and checking serial input, if anything is delivered.
"Anything" is delivered in form of single byte, once per second, minute or hour.

I'll try to verify the code without MIBAM and see whenever it causes the issues.

And if I switch to hardware serial input, will it still work with MIBAM?