After updating a user name in Active Directory, the people picker field still displays old information.
Edit the Form and delete/remove the old people picker fields having issues.
Publish the Form.
Load and edit the form in designer again.
Add the People picker fields back to the Form and publish.
When using the people picker field on a Form, if a user account has been added or modified in Active directory people picker will reflect the changes made to existing account but will also show old or stale information including the old account.