Cast on Freeze no energy on bosses

It seems cast on freeze wont generate any energy on unique monsters (bosses).
My max energy is 150 linked with hypothermia and frost bomb.
I tried it with with only one of them and other spells like comet.
It works on trashmobs so its not a gem issue.
But as you can see in the following video the energy stays at 66% and there is no trigger/-res, exposure icon on the boss at any time even tho they both get frozen.

Boss fight:
https://youtu.be/rzrx_Qbu06Q

Edit: Did some more testing to see if its an energy overflow issue.
In this test i had more max energy than energy gain per freeze. Still no change.
https://youtu.be/kOCZmd5gXAM