There was a change in the database table (BDItemUpsellLinks) from v9.1 to v9.5 & later. V9.1 software was enhanced to take care of the problem of UpSell/CrossSell items created in v9.5+ displaying on v9.1 sites. However, UpSell/CrossSell items created in v9.1 may not display properly on v9.5+ sites.
If you have a v9.5+ site that is not displaying UpSell/CrossSell items created in v9.1 correctly, please do the following:
On your v9.5+ site
Login in as Admin
Click on System Management tab
Click Reset Defaults
Select Converted old upsale/crosssale data to new upsale cross sale link
Click Delete
Restart site