Because osmosis isn't an actual strand weapon. The weapon itself needs to be strand, not temporarily changed by perk
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The moment osmosis activates it becomes a strand weapon so it should obviously activate all strand related perks. Thread of propagation fragment, volitale rounds from echo of instability, ember of empyrian, all of these works when an osmosis weapon is turned strand/void/solar. Not to mention threaded blast, another strand weapon perk on artifact also works. This bug is only a thing with those 2 artifact mods.