Sorry for the delay in answering you. This bug is strange, it’s the second time I hear about. I think it can happen if the character is too fast (but really too fast), then walls cannot process the collision. Thanks for reporting, I think I can solve placing a force outside the screen pushing him to center of the screen. Sorry for the problem, you have to reload the page and start from begging =/.
No worries. I'm delighted to have heard back from you regardless. Yes, this is a common quirk of many out of the box physics systems. Considering that it's a rare occurence, if you could merely detect when something like this has happened and automatically end the round, I think that would be totally fine. And I recommend this just because I imagine it would be the easiest band-aid solution. Infinitely better than a soft lock. In any case, I wish you the best.