Bugged out for me as well just then for the second time through >:(
Ticks off the ‘clear beach of undead’ objective but won’t go any further
I think Kesarai’s point might be true, because both times I’ve done it Tonn has blown the third tower before I killed the Veteran Preserver.
As a side note, this quest sucks. The NPC pathing is terrible and you spend forever just willing them to come to the next objective. I’m sure a lot of people wouldn’t mind if you just took the mission out of the game entirely (hint hint)