Hi apache
I've just tested it on my staging site and for me it works for multi-suppliers and setting a specific supplier. In order to keep investigating maybe some screenshots will be useful, or you can 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/
Remember to make a reference to this post.
Best Regards,
Andrea.