If a user was previously assigned to a role that has been deleted, then that user would not have a role assigned to them on their next login.
The ROLE_ADMIN was copied to modify permissions. A user is assigned to this copied role, let's say ROLE_ADMIN_COPY.
This role gets deleted, the user will show no roles assigned to them:
To fix this, simply re-add them to an available role.
To add them to an available role, you must have the following permissions:
Under Settings → Users and Permissions click the user you need to correct. In the box on the right hand side, select the new role and click Save: