I did several Dolmens today in the Imperial City Districts and only 2 spawned a chest I could loot. For most, after activating the pinion, no chest appears.
i was the only one who did it, i dealt all the damage. The dolmen itself has a cooldown so why would the reward chest have a seperate cooldown which takes even longer? If the dolmen respawns, it only makes sense that the reward for completing it should respawn as well.
I've solo'd these in quick succession and not gotten a chest to spawn on more than a few occasions; just what I've seen - put in a bug ticket with ZoS if you think there's a problem; I'm not a support rep.