When GlobalProtect users try to log in from their clients using their username, ip-user-mapping shows up as just the username instead of domain/username.
Make sure that the authentication profile in use for GlobalProtect has Netbios domain added in the domain field.
If you are running a verision prior to PAN-OS 7.0, the domain configuration is located under the server profile used in the authentication profile. Go to Device > server profile > LDAP/RADIUS/Kerberos).
When running PAN-OS 7.0 and above, the domain configuration is located under the authentication profile.