Skull splitter skill missing bug?

I’ve activated duck’s skull splitter but the skill icon didn’t appear amongst duck’s skills.
I’m using his crossbow and I saw on youtube that it’s supposed to work with that weapon.

I’d attached save game file but I don’t see such function on these forums. Is there anything else I can do?

Just to be clear, you are aware you have to choose which skills are active before combat, correct? You choose a major (hexagon icon), minor (square icon) and passive (round). I feel like maybe you chose knee shot first, which made it your active minor. Then you chose skull splitter, but didn’t switch it to be active.