Stun mastery dont work correct

Stun Mastery's Hits against you Cannot be Critical Strikes if you've been Stunned Recently doesn't work properly, and even if stunned, a character can still receive critical hits.
In the video, I'm using an Ultimatum where monsters always attack with cold crits. I first equip and then remove an amulet that converts all cold damage to lightning. While I have this amulet on, I'm stunned as expected, but when I remove it, monsters continue to crit, despite the Mastery stun and the fact that I should be immune to crit.
https://youtu.be/bsVRt2BVGSU

seaman#4993, Stun Mastery's Hits against you Cannot be Critical Strikes if you've been Stunned Recently: fairly reliable uptime, requires caution as certain modifiers that guarantee crits (Ultimatum mods, Ursine Funerary Altar) + modifiers that can cause you to take cold damage (e.g. map mods, Eater Eldritch Altars, or Taste of Hate) may make it difficult to find an initial stunning hit.

I have initial stunning hit.