We've encountered a bug when using the multistore mode when creating products with combinations via the webservice and managing their stock - it looks like the stocks_available table isn't updated with a new entry when creating a new combination.
Basically, when the multistore functionality is enabled and share available quantities to sell is enabled on the store group, the stock_available entry is not created in the database when creating a combination for a product.
This prevents us from updating the stock, since the stock is combination specific and we don't have any stock_available entry to modify for that specific combination. The webservice also disallows the POST method for the stock_available endpoint, so we cannot create it ourselves.
I've added the steps to reproduce this bug below - please let me know if you need any additional information.