I made this campfire, and whenever the player enters it, he is supposed to lose health; however, he loses health when he is not in the campfire and his health stays the same when he is in it.
Try changing your Overlap Begin Event and End Event to something like this. It should then work.
One quick suggestion is it is always a best practice to check your reference before accessing it. For example, in your Tick Event it will be good to check if your First Person Character reference is valid or no by using IsValid macro node. For the example you posted its not required but generally its a good practice. Saves null access crashes. :0
Looking at the screenshot you posted, the character will continue to take damage on tick for 5 seconds after leaving the fire before the boolean is set to false. If you remove the delay, you should stop taking damage immediately after leaving the fire.
I appreciate you.