Blame is an interesting word choice.
The bug is in Crate’s system. Modders cannot fix it. The best that modders can do is choose not to make things that interact with the game’s systems in this manner.
Item skill modifiers to secondary skills, such as Spectral Wrath in Necromancer or Torrent in Shaman, DO function and DO correctly modify the secondary skill as described. It’s merely a display issue on the tooltip generated by the game that suggests they’re applying to the base skill. Worse, sometimes this flips back and forth; it used to be that the tooltips would read correctly, now, they do not.
Again, for this issue to be resolved would require legwork from Crate. It’s out of our hands as modders to address.
What modders can do, however, is attempt to clarify these interactions via text elsewhere on the item, such as what Crate is doing on some of their items for Fangs of Asterkarn:
It does fall on us to make these interactions as clear as we can in spite of the wonkiness in Grim Dawn’s engine, but nothing will ever be perfect.