So, here's another couple of things:
I wondered if the problem might just be occurring with a full "power off" power cycle rather than just a reboot, so I have tried both three times and the issue occurred with all three full power cycles, and two out of the three restarts.
I then went into (PA) Settings > Library > Scanner and disabled USB Disconnection / SD Card Mount, but the problem still remains.
I know nothing much about the technical side of these things, but it does seem as though Android 11 may be unmounting the SD card when the phone is shut down, and this may be causing the issue with PA, but that doesn't really explain why it's been fine for the last two or three weeks, and the problem has just now reoccurred.