I believe this is normal in People Planning when register data is cleared. Unless a method (a small business rule and data management step works) is built to purposely clear data from the XFW_PLP_Plan table, it will stay there. Most likely the register data is getting cleared manually by someone or via a business rule.
@RevThere is no straight forward place to find how it got cleared. My starting point would be -
1. Cick on Register Audit logs
2. Filter on Object -> Register , Action type -> Delete
This will probably tell you who deleted the register.