Weird glitch that's happened once before: can only delete text when editing post. (not at charlimit)

I've encountered this once before. It's an odd error.

I posted this comment, then went to edit it.

I was able to press backspace and delete text, but I could not add anything. The comment is well below the charlimit, and it doesn't let me add text after I delete a bunch. Maybe it's the charlimit system causing the bug, but it's definitely not a charlimit "working as intended."

EDIT: Looks like it might be charlimit-related after all. My comment was 4461 characters long. I regain the ability to add text when I cut it down to 4055 characters. But wasn't the charlimit removed? And was it ever that short? Might this have something to do with how quoted text is counted?