In sitefinity you can find two "Form" widget. One of them with MVC label in the icon, one without this label.
I am easily reproduced this problem with MVC form widget,
But, i can't reproduce this problem with webforms form widget.
As temporary solution, i can suggest you to use webform form widget, if it is possible in your case.
Looks like a bug of MVC form widget. You need to contact with Progress support team and report this as a bug, because i am not Progress team.