There seems to be a bug at least in the Coffee Corner. I've been able to recreate it consistently with several comments in multiple threads.
Click Edit under Actions.
Comment text disappears and is replaced by a new comment field:
Fortunately, at least it doesn't lose the original text. But it's not feasible to edit any comments now.
I'm using latest Firefox and Windows 10 if it matters.
P.S. Just checked and it seems to work fine in Q&A, so looks like just CC is affected.