Role Display Issue

We have an Issue in SAP Portal with regards to the role. We have multiple roles for certain user (ESS, MSS, HR Approval etc). Most of them copy of standard or standard SAP Role.

Randomly we find that user's role is displayed as NULL instead of role name. The solution is to remove Portal group and add it back and everything is back to normal.

Has anyone faced something similar? Or Any known solution for this problem?