This is now my second customer to show up with this issue after an upgrade to 2021.  One had modifications from APS and WSP, the other was completely vanilla.  Somehow, the tabs from two entirely different modules/screens were merged together.

In the first case, all of the Customer Maintenance fields were merged on top of the corresponding tabs in Vendor Maintenance, and in the list of panels for Vendor Maintenance you could even see an OCUSTOMER panel!  

In the second case, the Sales Order Header tab was merged into the Purchase Order Entry screen - see this screen shot from when I first opened it up in Customizer after their upgrade.  Notice it says PURCHASE ORDER at the top, but you can clearly see SO fields like IT User ID, Sage CRM Opportunity, No of Ship Labels, etc.

Um..... WHAT???