i have done some testing today and it looks like I can get chest revards from world boss only on one character. If I try to participate in world boss event with any other character i get reward only for event (karma, exp, coins).
for example:
I killed tequatl with my thief and get normal chest +bonus chest after couple of hours i killed tequtl again with my ele and didn’t get any chest.
Then I try to kill shatt with my warrior for the first time today and didn’t get any reward not normal or bonus chest.
It looks like i can get chest reward only on my thief and not on other characters.