I’m trying to use Onatel on the magic night stages in Season 2. Buffs and debuffs should last for one turn, so I believe Onatel should still steal 25% of generated mana in the turn directly after her special. However, it appears as if she doesn’t steal any mana at all.
I use Onatel on these stages, and for me, she steals 100% for one turn. It is awesome if you can always hit the boss with enough tiles to refill your special and cast again, because the boss never gains mana. But if you ever break the cycle and can’t find enough yellow tiles to match, it puts you at a major disadvantage if you are counting on her to keep the boss from going off. Maybe bring mana potions to counteract.