MRP generates PR with vendor that is not assigned to source list or info record

Hello,

Came accross a strange issue - MRP has generated a PR for a vendor which is NOT assigned to the source list (there are about 4 others which are), and does not even have an info record created for the vendor/material/plant combo.

When I go into the PR that was genreated by MRP and click on the "assign source of supply" it then changes the vendor number to one which is on the source list.

How is MRP generating a PR with a vendor that has no source list or inforecord ?