Similar to Grinder not dropping a chest (but perhaps misunderstood?)
My first time leveling the grinder, I did not get rewards for tier 1 / tier 2. I then stopped and farmed at a later time (thereby logging out/in) and it now works fine. No chest appeared. I didn’t know, I thought you had to fill it to max at the time.
Later a guildy also filled theirs from tier 1 to tier 3, no reward. Also their first time.
Not sure if it’s related to being your first grinder drop or not, but it’s definitely a bummer to receive no loot.
Relogging, going to graveyard and running back, etc, did not fix this issue.