There actually is a guaranteed way to solve it.
Go to an instance where it hasn’t been completed recently.
lol yea thats kinda hard to do since it puts you in instances at random, however, you can up your chances by swapping to a difficulty most people don’t play. hard, expert, t2, t3 seem to be the least busy ones. Or logging on during off-hours.
It seems like the issue is that if one person in the instance completes some of the tenets, it completes it for all players in said instance but it only lets the original person have access to collect the tenet. and then lord knows how long it takes to reset the thing for the instance.
By swapping to a difficulty less used, theres a higher chance that the tenet wont be completed in any of the instances. Also playing during off-hours is the same solution.