I am currently using SharePoint 2019 and I have synced all of my users from AD into our SharePoint environment. When I add the Peoples Webpart on a new page for testing purposes and then add one of our users to it, only the email and the mobile phone are displayed when hovering over each profile. I would like to figure out why it doesn't display properties like department or their desk phone number. Is there something I have missed during the syncing process? All of this information is in AD and when I edit the profile in SharePoint.
I have the same experience with you.
As trevor said, People web part in SharePoint online and SharePoint server 2019 has different behavior, which has limitation in server.