Thank you @Sp1ke , I managed to figure it out in the end 
Topic summary
Whether products must be “Tracked by Shopify” for API updates, specifically inventory updates. Initial reply said tracking is unrelated to API changes. However, when attempting to update stock via API, an error occurred: “Inventory item does not have inventory tracking enabled.”
Key clarification:
- Inventory updates through the API require inventory tracking to be enabled per variant via the inventory_management field.
- Valid values: null (not tracked) or shopify (tracked by Shopify). Updates to inventory levels work only when inventory_management = “shopify”.
How to fix:
- Send a PUT request to the product endpoint (e.g., /admin/api/2023-07/products/{product_id}.json) setting variants.inventory_management = “shopify”. A JSON example was provided in the thread.
Outcome: Issue resolved after enabling tracking for the relevant variants. The thread confirms the requirement for “Tracked by Shopify” when updating inventory via API; no broader documentation link was provided.