I'll try a new form. What I do in the current form is position the form at the bottom center of the screen at FormCreate (for easier signing on a tablet is my thinking.. that might change). I also set the pen color and width also in the create event. Other than that, nothing else is done until they click the [Accept] button. I did try putting a AdvSignatureCapture1.Clear in the FormShow event, but at first I thought that resolved it but eventually it still did the diagonal line thing. The control is also on a TPanel, FWIW.
Form positioned using this line of code:
Self.SetBounds(
(Screen.WorkAreaWidth - Width) div 2, Screen.WorkAreaHeight - Height,
Width, Height
);