Occasionally, when toggling the Onion Skin on or off a drawing or action that has just been made will be un-done.
For instance: Draw a line on the canvas. Open the main menu. Click on the Onion Skin on/off toggle. The line that has just been drawn will disappear.
This only happens now and again, so it's hard to repeat - but it has happeed often enough for me to know it is a bug and not user error.
Customer support service by UserEcho