Hello Sara,
Check if this is happening only to business roles or all other roles?
Also, Go through these SAP Notes which might help you.
2119407 - UAM: Incorrect validity dates when business role is added in the simplified access request
2042631 - Validity Dates for Business Roles
1979538 - Simplified Access Request Role Validity Dates Validation Error messages
Check if this thread gives you some lead to solve your issue.
https://scn.sap.com/thread/3659245
Regards,
Deepak M