Culling strike not working through energy shield?

It is possible that I'm not understanding something about the mechanic, but I encountered the following:

Fighting Yama the white.
Enemy was certainly in cull range, very close to 0, to the point where I could not even seen the health bar.
Before I used cull, Yama recover ES to full.
When using cull, Yama was not culled, but instead the skill did regular damage to the ES.
It then appeared that when the ES was down in the 5% range that the cull work. It is possible that it was just regular damage, but it seemed like a cull at that point.

That is not my understanding of how cull works in POE1 or reading the tool tip. I'm fairly new to POE2, so maybe missing something, maybe this boss can not be culled or it is not supposed to work with ES? The cull seems to have been working otherwise on other mobs, unsure if I have encountered this elsewhere. Source of cull is Culling strikes 1 support. I'm using it with Boneshatter. Other supports are enduring impact 2 and overreach. I don't think it is to do with accuracy, as I am using a jade club, have 100% accuracy on the tool tip, and can see the skill was connecting.