Hey @H36615 thanks for getting in touch. My understanding is that inventoryAdjustQuantities only allows you to update a single inventory item and its quantities. Would you be able to share the GraphQL mutation you are using that allows inventoryAdjustQuantities to update multiple items? I tried to test this on my end, but the way the argument is structured should throw back a type mismatch error if you try to include multiple items as the variable input.
Hope to hear from you soon - let me know if we can clarify anything on our end.
Al | Shopify Developer Support