Setting PO Vendor on Sales Order

As from today, we have been experiencing a situation where we cannot Select a Different PO Vendor on Sales Order anymore in our Production Account.

We only have a Drop Down in the PO Vendor Field where we can now only select the Defined Preferred Vendor on the Item Record.

In our SandBox account we can still enter an Item on a line, the PO Vendor defaults to the preferred one but I can edit this field and change it to something else that is not even defined on the Item Record.  We have been working like that for a long time and this morning all of a sudden it stopped working :h_a_w: