I solve it, if you go to the 2nd one then the 1st and then the 3rd u can complete it
This bug still exists as of 2/12/2020... took me an hour to get through but this works - start the instance, don't talk to Tonn. Clear every monster ESPECIALLY THE ONES STUCK IN THE BASE OF THE TOWERS, then talk to tonn and follow through to completion.
Reason for bug (I think) - there are some monsters that spawn inside of the base of the tower that you can't see. If the tower explodes before they are killed Tonn will not move from tower 2.