It might me a race condition to loading and detecting ground collision. I noticed when creating a character i didnt have exit jump loop working that player would be in a falling state starting mission. My only guess is that calculation for collision was not completed yet. Also note that this same thing can occur when switching pawn ownership. pawn seems to have some kind of twitch that seems to be start of jump animation. Its a quick blip when it happens but its happens. Maybe a solution would be to add an initialization time delay before initially calculating any animation data for your Anim BP.