unable to complete termite mounds cave

i've tried my best to do this cave but it's bugged & nothing i've tried has worked

>open the quest book, select the cave so i can get the xp reward
>go through the cave, getting the two chests
>get to the final part, kill the guy that unlocks the end chest
>quest still visibly open
>open the end chest, get the loot, close the chest and the quest disappears/cancels instead of giving the xp chest like normal
>try re opening quest? nothing happens
>exit the cave through the normal exit w/ the quest reopened, still gives no reward

what should i do? it's not the end of the world if nothing can be done, i don't really care, but if there's something i'm doing wrong or if there's a bug,,, yeah

 

there are several entrances to the cave that all join after their first chest. you need to get the chests at the end of all of them

 

There are 3 entrances and 4 chests in total