[@Grimhildr] It's possible that you're getting a Javascript error which will stop further Javascript execution on the page (which may include the Javascript that paints and controls the editor).

Could you open your browser's Javascript console, then try to replicate the issue and tell me if you see any errors pop up?

That's the best way for me to figure out what might be going on.

But I'll take a look at the code changes I made anyway over the last 2 weeks to the editor. They include the post-revision system and the autosave system, so one of them may have introduced a regression that only shows up for a few people. :(