I was trying to mega evovle my Charizard, I did have a little trouble figuring out how to do it but I did it eventurally, and when the battle ended my Mega Charizard was just a normal Charizard again. I was fighting very underleved pokemon and oneshoted the pokemon i was fighting. But if you say they're suppose to turn back to normal when the battle ends then i must have done it right. I just thought they would stay mega evovled.
P.S. sorry for my bad English.
Edit: I haven't played any of the original pokemon games so i don't know how mega evelution works xD.