Hi @NinaD1,
If I understand it correctly 'User' is a SharePoint Person column and you are using the Power Apps browser editor. What happens when you press 'Add Field' in the side pop-up - no columns are displayed in that pop-up? I have provided the steps below as a reference.

Select the combobox / DataCardValue and go to the right property panel.
- Select 'Edit' in the Fields section
- Press 'Add Field' which should open another pop-up menu
- Select the column to be displayed
- Lastly, select add in order to use the selected column as column display field
As a note for Person Multi-Select fields: once I selected a display column, it seems the combobox did not display any information even though for that item multiple people were selected in SP. I was able to resolve this issue by adjusting the combobox DefaultSelectedItems property from [Parent.Default] to Parent.Default (removed the square brackets).
In regards to a single-select person column: I unfortunately am unable to replicate the issue on my end. After indicating the display column, the correct selection (& respective column) was displayed. Please note however that, as of the time of writing, the modern Form control is still in preview. You can always opt for the classic 'Edit Form' instead and style it accordingly.
I hope this helps!