I think this happens because generally, autocorrect will do its thing after pressing space, or some other type of white space. At the end of a message, we usually press enter, which then serves both as the autocorrect trigger and the send trigger. You could notice that autocorrect has still selected the word (thus indicating its intention to correct), but really, that’s a bit subtle.

I would think that pressing enter should only send if no autocorrect was triggered by it (and sending requires hitting enter a second time), but maybe then you get the even bigger problem of constantly forgetting to actually send.