I have been setting up Microsoft Identity Manager fur our new 2019 environment and ran into an issue where imported users profiles would not use the netBIOSName under their sharepoint profiles. I have went through the steps here : https://blogs.technet.microsoft.com/spjr/2017/10/08/sharepoint-user-profiles-are-imported-with-wrong-domain-name/
and still experiencing the same issue.
the fqdn would be domain1.domain.com and netbios would be DOM1 the user in SharePoint shows up as domain1\user instead of DOM1\user
At this point I am not sure what else to try, all the other domains in the environment sync fine as the netbios match the domain.