Seems to work with target dummies, maybe this was an isolated issue with the mobs I found?

Hey, sorry for the late response.

I looked into this and found that the function that checks if the skull icon should be displayed was using the older UnitLevel function, that retrieves the unscaled level of a unit. Instead of the relatively newer UnitEffectiveLevel which the level function uses. Basically the scaled version that fetches unit level was saying this is a skull unit, but the older one was retrieving an actual level.

This should be fixed in the latest release.