You might recall this from the rules for captions topic:
FROM NOW ON CAPTION POSTS ARE LIMITED TO 275 CHARACTERS.
And there's also:
THERE IS NO CHARACTER LIMITATION ON THE LENGTH OF THE SET UP (well, within reason and within the forum's guidelines). ONLY RESPONSES WILL BE LIMITED TO 275 CHARACTERS. But as there really isn't much need to repeat the entire set up in responses, 275 characters should allow much more than enough leeway to complete a phrase or fill in the blank(s).
it appears that MB chose to help you by trimming your caption instead of removing it outright.
That's exactly what I did.
In fact, rather than remove them outright, I've edited several completions and fill ins for going over the 275 character limit. The code to display a warning is in place for the Caption This! boards, but I haven't gotten a chance to code it for this board.