One user was created in SFDC with username as firstname.lastname@example.org with id as ID001.
Whenever you sync the user data with it will create a new user and SFDC User ID will be updated as ID001
Now, the customer changed the username to email@example.com and inactivated the user
and then created a new user with the same username as firstname.lastname@example.org with id as ID002.
Now if you run the sync SFDC User ID in MDA user object not updating as ID002. Because already we have a user in MDA with the same username.
So the new user unable to access the Gainsight data as he was not loaded with new SFDC ID