I apologize for any inconvenience caused. I found a weird behavior with inbound stock but it is not exactly the same as yours. When I create a PO, when it is pending status, the inbound stock is not updated in Stock Central, but when I pass the PO to Complete status, the stock of the product is properly updated.
So, please, could you tell me in which kind of product you are experimenting this issue? Which ATUM add-ons do you have installed? Could you try to deactivate all plugins except ATUM add-ons and Woocommerce and check if the problem persists?
In any case, if you still experiment the issue, I have to ask you to open a private ticket and provide us with your website credentials in order to take a look. This is the link to open the private ticket: https://stockmanagementlabs.ticksy.com/