I've something similar on an Edge 1000 on the last couple of 200s I rode, though at the start of the ride rather than mid-way. My position was 'stuck' until I cycled screens, after which it would update and then stick again, and there were no TBT instructions.
I put this down to a bad GPX file that I've since updated (but have yet to try), but thinking about it now, a firmware glitch seems more likely. Not sure what version I was on when this first happened a few months ago, but I'm now on 15.50.