devilcoredev/TrinityCore — comment on commit 1fb98b962f8909f048a7c8dd12fc5d6e899fbbaa
File: src/server/game/Entities/Creature/Creature.cpp line 1655
By devilcoredev:

So, if we do not change this line:
if (GetCreatureInfo()->MechanicImmuneMask & (1 << (spellInfo->Mechanic - 1)))
bossess are immune to grip and taunt effect for example for the Death Grip.

If we change this line bossess are immune to root mechanic sometimes for example.
There should be a extra flag that if it's enabled we should search also on mechanic.