Thanks brundlefly, that seems to have done the trick
Enabling 'Set Now Time with full reset' causes a pause in audio playback when toggling the loop region, so wasn't what I was after.
Changing the playback I/O buffer size looks like it resolves the issue. I had it set at 512KB; moving it up to 1024 made no improvement, I knocked it down to 256 and the audio after the end of the loop point plays back correctly after toggling the loop mode to off.
Bit counter-intuitive that reducing the buffer size will improve playback performance, but glad to get it resolved. I don't know enough about it really, but it does smell a bit like a 'feature' - audio playback getting corrupted after removing loop points, when it plays back fine if not looped, doesn't seem quite right.
Have also managed to convince myself that this didn't happen previously - I might try rolling back a few versions and see if I get the same issue when I have time. Then again, I can convince myself of most things given enough slack...
Thanks again for the help.