If an entity (A) is attacked by another (B), B will be the attacker of A for 5s. That's how it works.

(And if entity C damages A, it replaces B as A's attacker).

But if A is damaged by another source than an entity, like a fall damage, a fire, a cactus, ... Then the attacker is not reset and we still detect B as the attacker of A.

If this is noted as "working as expected", which can be justified. Perhaps we could have a new argument for "/execute on": "victim". That way we could know who the victim of the entity's last attack is. Because "target" does not work on players.