Well, right now im almost 100% sure that it does have to do with sneak attack because I did have sneak attack paried with contain. I changed the paired magic, and it changed what happened, although Tifa still attacked first and i still got the "Tifa was caught by surprise" message. Ive never seen that message on any other playthrough before though. And no, I was not cheating, and it started happening in almost every other battle or so, but it was at random.