A few months ago I had an identical flap appear under my tongue (and I do mean IDENTICAL, it was even on the same side of my mouth). At the time I was quite concerned and was on the verge of making an appointment with my doctor when, after about a week, it started to become less noticeable. Within 2 weeks from when I first noticed it, it had virtually disappeared. It's now as though it was never there.
I remembered this post from when I was trying to research what it could be and thought I would share my experience in case anyone else should come across this in future. For what it’s worth I’m a 38 year old male.