I understand. 
The issue of your current implementation is that the pawn it’s not holding a reference to the actor its checking against.
IMO the spawn actor is the one that should apply / remove the buff to the actors that step on it, it should not be in the pawn.
If you are open to suggestions, I would try something like this in the actor that is supposed to apply the buff:
If you need a timer for each pawn that exits the buff area then perhaps the buff logic should be in a component that gets added when every actor steps on the buff area. The buff area can check for the component to remove any timer or any other logic for stacking buffs, etc.
I cannot help with replication if that is part of your project. I’m sure anyone else can be of more help.