There's a 2 turn Dec Speed on the enemy debuff bar there. 

Did you mean to include a different screenshot?

I think we're seeing the same thing here. 

i

Yes I meant this screen that debuff was already there from the previous turn. In the included screen you can see he double crits with no debuff placed or resist text.

He can't place a new Dec Speed buff when there is already the exact same debuff on the bar.

You sure he isn't hitting while there is a 2 turn speed debuff up already, like we see in your screenshot, then the Robots clear all the debuffs with their passive? This would make it seem a lot like he's bugged, although it isn't. I can see in your screenshot Kurzad is already running away from the hits, and there is a still a2 turn Dec Speed debuff in that moment.

Here is the Robot's passive, for clarification: 

Retaliatory Launch [Passive]
Whenever the Iron Twins’ HP drops below 85%, 70%, 55%, and 40%, instantly attacks all enemies, then removes all debuffs from the Iron Twins. 

So I can easily see Kurzad not applying his debuff since it already is applied, then hitting a HP threshold for the passive, then all the debuffs clearing.

i

Trying to clarify what I'm saying here...I'm referring to circled part, it would have text with the debuff and icon or green resist text regardless of the debuff already on the boss. You can see neither is present.

I'm out of keys for the night so I can't do anymore screen shots, and deleted the ones that show the other two normal variations. Can repost tomorrow if want/needed as it seems pretty consistent after watching a few fights it happened about 2x per fight.

As I said above, he cannot apply a 2 turn Dec Speed debuff when there already is one on the bar. It's just the way the game mechanics work.

Okay that makes more sense I guess. I thought regardless of the debuff existing it would still be placed/refreshed.

I wasn't concerned with the boss skill, I just noticed that I was not placing or resisting the debuff. 

Edit: Also I blame master hexer mastery for my confusion. All blame can go there thanks.

Okay cool. :)

If there's only one turn left on the Dec Speed, it should refresh without issue, since it will add the extra turn to the debuff. If you catch it not doing that, please ping me.