Not sure if this has been raised before, however, it seems as if sometimes when people respawn their FF doesn't go away or if you join while someone has an FF it doesn't go away? Not entirely sure on how to reproduce however I got a clip of it:
www.youtube.com/watch?v=hayRyQq0vd0

i've learned that if you see that glitch you can still kill the player

Yeah, it sometimes happens