Comment 2 for bug 1473246

I'm hitting this exact same issue with a different device, it's a development board that communicates over USB CDC (an Adafruit Bluefruit Micro, a little bluetooth LE microcontroller board). The device has similar behavior as the mentioned MediaTek preloader where it implements a very basic USB CDC endpoint and can abruptly disconnect at times if for example it times out waiting for a response from the host computer.

Not sure what else I can add unfortunately to the discussion but your debugging info is super helpful to confirm this appears to be the same issue. I'm not familiar with ModemManager either but is there any way to rollback to an older version and pinpoint what change caused the issue?