Our team is having the same problem. We figure that when Blizz put in the fix the other day that it changed how the damage is being put to the tethered tank. The tethered tank appears to be taking all of the non-mitigated damage. So if you have a brewmaster monk, for example, the tethered tank is getting hit for all of their stagger damage (i.e. the unmitigated damage).
My group tested a lot of different combos of tanks, and this appears to hold regardless of the tank.
Our current strat is 3 tanks. When 2 tanks become tethered, the 3rd tanks taunts the boss to take the damage so that the other two don’t. The other two are just doing dps and helping gather adds during this time. This strat is has been working well for us thus far.