Indeed, this is an issue within this form which we have already addressed.
However, for the time until our next release, I can provide you with a quick workaround which you can use to overcome this undesired appearance.
I have attached a sample project which demonstrates how to achieve this.
Basically, I create a new Windows Form and inherit from the EditAppointmentDialog class. In the constructor of my custom dialog I reset the AutoScaleMode property of the form to None. After that, I subscribe for the AppointmentEditDialogShowing event of the RadScheduler and in its event handler I replace the default dialog with the new one.