If you’re doing something like stealing current from the on-chip regulator of an FTDI USB UART (or its clone) then browning that out with LoRa radio transmit current would cause the USB device to fail, and the OS to throw errors at any program trying to use it.
You probably can power the board from USB, but you should use the USB VBus and a distinct regulator, not the low-current output of a USB-UART board which can only spare a few milliamps.
It is also theoretically possible that a poor USB implementation could be sufficiently disturbed by radio frequency energy from transmission to drop the connection - not likely, but still possible. Consider trying other (shorter) USB cables. If you were using a hub, try it without, if you were not using a hub, try it with one, try a different computer, etc.
Ideally a serial line connecting to a radio device would have ferrite beads or small chokes to keep undesired energy from coupling that way; typically this is done to protect the radio receiver from computer noise, but technically it works both ways.
And please do clarify if you are using the EVB or your own USB UART solution.