The onion skin is still visible in the scene when gameobject is hidden.

2. How we can reproduce it using the example you attached
- In the Project menu navigate to "My_Animations" and drag one of the "Cut_artboard" spritemeshes onto the scene.
- Select Window\Anima2D\Onion Skin. Make sure it's enabled in the menu.
- Select Window\Animation
- Create a new animation for the cut_artboard you selected.
- Press record to add a key frame at the start.
- Add another keyframe a few seconds along the animation time line and move the cut_artboard to create a basic animation.
- When you scrub the timeline you should notice the onion skinning effect.
- Select the cut_artboard gameobject in the Hierarchy.
- In the inspector hide the cut_artboard by clicking the tickbox beside the gameobject icon.
- Scrub the timeline.
Notice issue >> The onion skin effect still appears for the hidden object.