Fire Ele bugged didn't receive chest or bonus
If no damage numbers cam up then you weren’t hitting it but that doesn’t matter. Did you get credit for the event? At least a bronze?
I haven’t experienced the issue myself at Fire Elemental but at the Maw and Jungle Wurm it’s pretty easy to have them killed before you can even deal enough damage for credit when there is a lot of people there. The wurm can die before getting out of the ground. The svanir shaman can die in the time it takes to run from where the 3 portals spawn to where the shaman is.
Also make sure the Fire Elemental event isn’t the ONLY thing that character is doing.
It happens to a lot of players:
https://forum-en.gw2archive.eu/forum/support/bugs/World-Boss-Bonus-Chests-Not-Always-Appearing/first#post1858712
https://forum-en.gw2archive.eu/forum/support/bugs/New-Boss-Chests-bugged-after-changes/first#post1822157
https://forum-en.gw2archive.eu/forum/support/bugs/Not-getting-daily-chest-from-world-boss/first#post1787798
It has to be a bug! :-(