This was strange. Which of the values do you see if you look at the record in a list view?
Is the user a normal AD-user or an external user?
Does the display form value stay the same if you save the other value in editform?
What do you see if you hit F12 to bring up the developer tools – select Console and type in this:
Expand the value by clicking the little triangle before the value.
Please note that you must use the correct internal name for your field – AssignedTo might not be the correct internal name. To see the field name go to list settings, click on the field and look at the URL and you will see &field=Internal_name_of_field