That doesn't mean it doesn't have hack checks. Does it have good ones? Not really, but it definitely does have hack checks. Try to deposit an illegal Pokemon and you won't be able to; a glitched form isn't indicative of anything. The game you transfer that Pokemon to is going to revert it anyway unless you use
cheats or hex edit in that game. Pokebank isn't recognizing it as a mega regardless if it's stuck using that sprite or not.
Just for legitimacy sake could you take a pic of it showing "M. Charizard" or whatever in your Pokebank? I guarantee it would just say "Charizard" not "M. Charizard/ Charizard X/Y" which proves the bank doesn't recognize mega evolution because it shouldn't.....