Not sure if you are familiar with the Hotfixes that can be applied to the application, but this sounds like a problem we already resolved. Please go to GTAC and download the latest HFs. You will find on the Tecnomatix_12.1.x_Hotfix_Master_List.xls the following issue:
HF 38 - Parts got renamed back to default name after reloading workcell
Typically we apply HFs in order, so install the one before it first.