The RP2040 USB lines are very sensitive to RF noise - that means that if you are using the radio module at full power and you have the antenna near the board, say because you are using a spring-coil or plain-wire antenna, you may see the USB subsystem disconnect or act flaky. 

Here are some solutions to try:

1. Use a uFL to SMA adapter cable, then have the antenna far away from the board as possible. Don't lay the antenna near the Feather!
2. Use lower power for transmission on the radio module. In circuitpython, use the tx_power property and assign it something like 5 (lowest value to start) and work your way up! See https://docs.circuitpython.org/projects/rfm9x/en/latest/api.html#adafruit_rfm9x.RFM9x.tx_power
3. Try a lower baud rate when using USB serial. Lowering it down to 9600 worked here.

This only seems to affect the USB system, so if you're battery powered it may not be an issue that affects your hardware