Is this the first time you've noticed? They've done this in the past when bots go nuts with spam. For example trying to send every member some phishing scam link. By putting up a captcha they freeze the bots, and it seems it stays that way until the scam people go away.
This is the reason.
I believe there is a bug in the implementation, where if you have a mistake in the first attempt, every attempt after will fail if the original message doesn't get modified.
I've tested that a few times and it seems to be the case.
Call Grace Hopper!!
(Only a nerd like me will understand)
Maybe it's time to learn Morse code?