Decompose makes you lose mana with Archmage meta

Archmage: "Non-channeling spells cost an additional XX% of your maximum mana"

The Corpsewade boots trigger Decompose skill on step

Decompose is classed as a "spell" in the UI but you cannot support it with support gems that support spells. Only gems that support "any skill" can be socketed

So either Archmage incorrectly effects Decompose or Decompose is bugged alltogether