When a user changes the vendor, the system automatically considers that adjustments to the storage dimension may be done. This happens because vendors usually on Purchase order defaults may have default site/warehouse settings. So, when the defaults site/warehouse is linked to a vendor and you switch from one vendor to another, the system auto-populates in the associated information.

Additionally, this behavior isn't limited to purchase orders, it's the same for sales orders as well. 

Suggestion: if Purchase order defaults are empty on the vendor card, then the system should not delete existing site/warehouse from purchase order header.