This is a product defect (TF00767291).
Steps to Reproduce:
1. Reproduce in environment with parent and child domain
2. Create a user in child domain and a user in parent domain
3. Open the web admin portal with admin credentials
and customize the logon display name in global settings and
set it to "msDS-PrincipalName'
4. Test logging on web interface with both of the users
and notice that the logon customization will not be in effect for the user in the child domain.