I would just add a print string after the "set is Hit" node and look what happens, because i could imagine that the "is Hit" variable isn´t updating properly. And if that shows no results I would add a print string infront of the timeline, so you cant keep track of what is happening. Becaus as I understand the Blueprints, the timeline is just used as a triggerable Event tick. And what happens, if you shoot another fire spell into the "broken Mirror"?