Vaal Temple Forge Master Stun Duration Nonexistent

The rare Forgemaster guarding get stunned for about 3/4ths of a second and are back up running you down at mach speed.

Mektul from Act 3 would get stunned and put him on the ground for maybe a second or so, why are the same archetype of monsters inconsistent? Is "effectiveness" affecting the stun duration of these monsters and is that effect poorly communicated to the player? Effectiveness stats a boost to monster life, experience, and quantity.

I don't understand why the monster behavior is now wildly inconsistent now