Why is ASCP collection process overwrites vendor data from another ERP instance?

We have a test ASCP instance (distributed environment) pointing to two test ERP source instances.  We noticed that a standard data collection (full refresh or targetted refresh) overwrites previous data collected in MSC_TRADING_PARTNERS table. We were addessing an issue reported by users that they could not select a planned order line, firm it, and source it to another vendor. The 'Source Supplier' LOV was blank.

In an effort to resolve the issue, we did a targetted referesh in standard collection for just the vendors / customers. It fixed the issue, but we noticed the table above lost all its previous records collected from the other ERP instance.