In this case, the Anchor property of the Separator is set to Left, Top, Right. When we close multiple times the designer, the width of the Separator is increased. A possible workaround here is to avoid adding Anchor to the right in the designer and set it in code.

public RadForm1()
{
    InitializeComponent();
    this.radSeparator1.Anchor = AnchorStyles.Left | AnchorStyles.Top | AnchorStyles.Right;
}