I raise this in case it sheds any more light on the USB issues.
I am using rp2-pico-20210209-unstable-v1.14-11-gc7aaee2b2.uf2.

I have a substantial uasyncio application developed on Pyboards and ported to the Pico. The test script outputs regular messages via print statements. The application maintains a heartbeat LED via a coroutine on the Pico.

It runs fine if I suppress all print output.

With print statements enabled it also runs so long as I periodically press a key on the keyboard (in rshell or in miniterm). If I don't, it runs for a while then stops. The heartbeat stops flashing until I press a key, when running resumes. The code has evidently stalled.

Unfortunately my attempts to create a trivial test case have so far failed. I have never encountered this behaviour on Pyboards, ESP8266 or ESP32.