The Siphon Enchantment does not place items in your offhand, even if you have the correct item in your offhand

When you pick up a dropped item, if that same item is in your offhand, the game will prioritize stacking that item with the one in your offhand. Unfortunately, if your tool has the Siphon enchantment on it, this behavior is different, it will place the item in a regular inventory slot even if you have that exact item in your offhand. It makes sense that Siphon's behavior should be consistent with the vanilla feature of picking up items.