X12S MPM communication errors

[Unb0rn]

I have a modded X12S (XJT replaced with ISRM, Bluetooth with PARA, externalaccessmod, inverter replaced to support full-speed port, touchscreen installed) it used to work fine with Multiprotocol module modified the way described in this issue. I haven't used it for a while and when I tried using it again - it failed with No MULTI_TELEMETRY detected error code. I made several updates between the last time it worked and now.
Now the radio runs EdgeTX 2.9.1 (Bootloader 2.9.1), ISRM 2.1.9 and I have two MPM modules - IRX4 and IRX4+, both modded to work with X12S and both running the latest 1.3.3.20-serial MPM firmware, both work fine in X9D+ 2019SE and neither of them work in X12S.
What I tried to do:

• Tested the JR bay to be working correctly - both ACCESS (R9M 2019) and CRSF (ExpressLRS, Thor and DIY) modules work fine, ExLRS modules are even able to run F1000 mode in 921k baudrate, they communicate well both with their LUA script and models, return telemetry good.
• Reflashed both modules in X9D+
  Nothing seems to help. Toggling internal ISRM on and back off switches the error message for MPM from "no telemetry" to "no serial input" and cycling through protocols briefly returns the correct module FW version.
• Downgraded the radio firmware to 2.8 and even 2.7 (didn't touch the sdcard or BL) - didn't help.

It would be great if anyone could try and reproduce the problem before I open an issue in edgetx repo, not sure if this problem systematic or only mine.

I also added a couple of comments in MPM repo here. Maybe it can help understand the issue better