This happens frequently. I compose a message, hit "return," see that my message has been delivered, quit Messages, and then, hours later, relaunch it and find
this, i.e. that the text that had disappeared from the composition box when I sent the message has reappeared...except for the final period which is always missing.
Sorry, but I can't date it, although I
don't think it's been happening since November when I began messaging.
Anybody else?