This was a bug with the popup that gives you some pre-made comments to post when deleting or recommending deletion. Since this functionality is only available in the Low Quality Posts queue, the bug only appeared in that queue. It wasn't consistently reproducible because you had to previously have had a review were you recommended deletion for this issue to appear.
That bug was fixed a week ago (it was another symptom of this bug), except for a very small remaining problem where only the very first click after deletion may do nothing, but only in very unlikely circumstances. This remaining bug is also fixed in the next build.