Can you confirm:
1. If you navigate to Inventory -> Inventory Management
2. Click the binocular icon to view the product details -> Choose on storefront tab
3. Is the checkbox for 'always sync available quantity' checked?
999 is the default value set for the available quantity field for each storefront as ecomdash uses the lower of Quantity on Hand and Available Quantity.
If you wish to not sync the value of available quantity, make sure the check box for 'Always Sync Available Quantity' is unchecked.