You probably are on this already. But just to be sure...
We have a few products that are available from multiple suppliers. Maybe more than one wholesaler, or the wholesaler and the manufacturer. We can get round this now by doing a PO for multiple suppliers when in fact we are just doing a PO for one.
That's clearly not ideal. logically, it should be possible to attach products to more than one supplier, each with their own supplier SKU. I see that would clutter the interface. Failing that, it would be great to add products from another supplier with the facility to manually add a temporary Supplier SKU or hide the original one. The main thing is that the PO should reflect where the order is going, and currently it doesn't.
One small thing is that when I try to tag a nameless PO with the name of the supplier in the Description, it does not get picked up by PO search, which is a pain.
I think this is related to your Premium plugin improvements but it seems to me a core plugin issue because the current behaviour is a bit off.