Applying a strong stagger attack or career skill to an enemy that is in a stagger immune state seems to render the enemy immune to weaker staggers until the enemy is hit with another stagger of similar, or greater, magnitude.
The simplest way to reproduce this is with Foot Knight’s Valiant Charge career skill as shown in this clip below.
As shown, all one needs to do is hit an enemy with the career skill as they are in their climbing animation. This renders them completely immune to lesser forms of stagger such as: being pushed with a shield weapon backed up by 650 Hero Power, Opportunist, 10% vs Skaven, Enhanced Power, and Comrades in Arms. Being hit with Valiant Charge again (or a bomb) thankfully breaks this state and returns the enemy to normal behavior.
Far from being confined to Valiant Charge, other large “boss scale” staggers are equally capable of triggering this glitch. Other sources of stagger that were tested and confirmed to to the same thing: bombs, fire bombs, Mercenary’s Morale Boost, and Bounty Hunter’s Locked & Loaded.
Additionally, many normal weapon attacks are able to trigger this glitch in weaker enemies in slightly bizarre ways, such as these following clips: First exhibit, a one-handed sword with Opportunist is able to make a Skaven Slave immune to the stagger of the light attacks, but not the push. Second exhibit, the volley crossbow is able to make a Skaven Slave immune to the falchion’s push and first two light attacks, but not the 3rd overhead light attack or the heavy attack.
Strangely, I was never able to replicate this glitch using pushes or “shield slam” attacks such as sword & shield heavy 1.