Cannot assign skills that are coming with a weapon to weapon-set-specific skill trees

I´m using the new Atziri's Contempt spear and want to use it with two separate skill trees - Weapon Set 1 for Spearfield (from an uncut gem) - and Weapon Set 2 for Shattering Spite (which comes with the weapon itself).

I locked the weapon for both weapon sets, and I´m also able to assign Spearfield to Weapon Set 1 - but I cannot assign Shattering Spite to Weapon Set 2. The context menu in the skills overview is simply missing.

Not sure if this is by design, but I could see great opportunity for additional build variety if this would be fixed(?).

Thanks!