An LDAP group had been imported to PAM. When the group was deleted, one of the users in the group remained behind in PAM w/o group association. Because it's an LDAP imported user, it cannot be deleted manually. There was no error when the user group was deleted.
Observed on PAM 3.1.1 but the same issue will appear on older PAM releases as well.
One user on the appliance has its "Email on Login" set to be the user which can't be removed from PAM .