shadow behemoth no rewards
It seems camping there with your ranger is your “problem”.
For more information look here:
https://forum-en.gw2archive.eu/forum/support/bugs/No-Chest-After-Shadow-Behemoth-Event/first#post2063832
And here a shorter version: https://forum-en.gw2archive.eu/forum/support/bugs/World-Boss-Bonus-Chests-Not-Always-Appearing/page/2#post2188923