This has been around for years. If you put to much DPS into the Bone Colossus boss, it will reset. It does not like to be burned down.

Also ensure any summons lapse and are not recast in preparation for the transition and respawn.

It may not be the sole issue, but it is a contributing factor to bosses resetting. Last time I ran it, we burned it very quickly and tried slowing down, but that didn’t stop the issue. We had it reset every single time like clockwork until our necro tank let mender lapse and did not recast it until after the transition. It worked fine when she did that.

We also had the issue on first boss of LC. As soon as the dps with the necro line stopped precasting summons, the resetting went away.