Yes, this happens because the drawing was created with a visible pen instead of an outline pen. Even if you try to change the colors using the outline pen, you can't hide the drawing. In order to fix it, you need to recreate the drawing you want to hide using the outline pen. I suggest you contact the developers for further information. You can get in touch with them using Customer.Service@Ashlar.com
and Support@Ashlar.com
email addresses.