[GAMEPLAY] Enemies become immune to Tahm Devour even with 3 stacks

I was just in a match vs a top lane garen, and again more than once I would 3 stack him, then combat happens, then before the stacks wear off I try to eat him and he does not register to devour.

Every time this happens, it seems to have to do with buffs on the champion wearing off. For example, if Garen casts W and it wears off, then I cannot eat him. If Nasus ults and it wears off, then I cannot eat him. It's extremely frustrating.

Also, I can still stun with Q, I just cannot eat with W. So the stacks are there. W is just not seeing them or something.