Hi Jono ,
Nice to see you around.
Jono Product Data > Variable Product > Inventory > Manage stock ticked, Stock quantity = 0, Allow back orders? = Do Not Allow
no reason to have the managed stock ticked here as you are managing stock per variation. untick the box, pls. I do not think it will help with your issue but may prevent others in the future.
Regarding the out of stock message, I am not sure why is this happening if all your settings are correct? Dont think is the Multi Inventory or ATUM issue though. I have done some WooCommerce issues google search and there were similar claims when other plugins got involved.
Can you try the following, pls?
1. go to your installed plugins page and select all. Deselect only WooCommerce, ATUM and ATUM Multi Inventory. Now press the deactivate button to deactivate all plugins.
2. go to your theme settings and activate a simple WP or WC theme like the storefront (WC) or twenty nineteen (WP).
3. clear your browser cache and check the product page. If it is still showing on backorder I will need to get a deeper look into your site.
- if it works fine - then start activating plugin by plugin while checking the front end. It will stop working after you activate the conflicting plugin.
Hope this helps.
Pavel