Are you using a non-unique ID user store?
Also, could you let us know the version of the IWA Kerberos authenticator you are currently using?

There is a known issue(https://github.com/wso2/product-is/issues/21053) related to non-unique ID user stores when used with the IWA Kerberos authenticator. However, this issue has been resolved in the latest version.

Providing the above information will help us identify the exact cause of the issue.