Is it intended that the weapon damage stay at 120% when investing skill points or is it a bug?
I think, it’s intended. Those WPS skills arent meant to have superior weapon damage, probably. They are meant to make it up with their added flat damage and additional effects (ADCTH/multi-target hit).
After all, WPS like Belgothian Shears and Makovian Advantage also dont scale Weapon Damage with level.
yeah there is no hard and fast rule that every weapon damage skill must get higher weapon damage % per skillpoint.
inquisitors’ storm spread is also one of the WPS that doesn’t scale (always 33% weapon damage, but some extra projectiles at higher ranks i think) - similar to necrotic edge (constant percentage, but more targets)