Looking pretty good, but it needs some fixes.
Event ActorBeginOverlap
Remove the ApplyDamage from the Start Closed of the Do Once, you don’t need it there. Start Closed should be unchecked.
After the DoOnce you need to set bApplyDamage to True, otherwise the WhileLoop will never be executed.
Add a Delay after the SET Max Health otherwise it will kill all your health very very quickly.
Event ActorEndOverlap
Remove the DoOnce, you don’t need it there
Out of the event execution pin, SET bApplyDamage to False. Then connect the output execution pin of the SET to the Reset input pin of the DoOnce attached to the Event ActorBeginOverlap