I call a Form from a Gallery containing a People/Group list item, I populate a ComboBox in the Form with that data. I have two Form labels that are datacard.selected.DisplayName and datacard.selected.Mail. The DisplayName datacard contains the users display name but the Email datacard displays nothing. If I choose another person from the Combo box then the Email datacard gets a value.
Can you share a bit more about your issue?
I made a test. There's no issue on my side.
Please try to re-create a collection and check if it is solved.
If you take what you have and call a form on another screen then look at the MAIL property, what do you get? My first picture represents your reply (all the data is there), my second picture represents the data in the form on another screen (missing MAIL data).