I get what you're saying, but some stuff doesn't add up.That shouldn't really matter because face affects everyone who uses magic, and dragons have no special resistance to it. If anything, Acnologia is the only character who is resistant to all types of magic. So if anyone had resistance to face, it would have been him.
Acnologia didn't revert to his human form, and had enough MP in his tank to roar at Igneel in their rematch. If face wiped out all of his MP, then he probably would have needed to rest for a day. Keep in mind that one of Acnologia's roar is more powerful than pretty much the entire cast. If he can recover that much MP in such a short amount of time, then his MP is effectively impossible to exhaust.
We know for a fact that Acno had already beaten prime Igneel back in the days, and stole his soul. Maybe he beat him because of soul steal to begin with, idk.
x791 Igneel, at best, had the power level of prime Igneel. The parent dragons wanted to go to future to begin with, because there wasn't enough ethernano to restore their power after Acno absorbed their souls or something. It suggests that parent dragons would've probably just been restored to their original powers instead of having a powerup.
Unless Acno got weaker in the 400 years, due to rust since there was no one to fight with him, it seems weird that Acno would lose to Igneel again. Because Acno did lose the round, Igneel could've torn his body to pieces instead of giving a speech to Natsu right there.