As of API version 2026-04, activating an inventory item with inventoryActivate without providing an available or onHand argument will no longer default to zero in all cases. For an item that was previously active at that location, any existing active and onHand quantities for the inactive InventoryLevel are preserved.
This is a companion discussion topic for the original entry at https://shopify.dev/changelog/inventoryactivate-now-preserves-available-quantity