Onion skin addon groups together every vector costume

[fupicat]

Describe the bug

Having the onion skin addon on groups every vector costume together once you open one.

To Reproduce

1. Enable onion skin addon (no need to toggle the actual onion skin button in editor)
2. Go to the editor and open the costume editor.
3. Move a vector shape.
4. Open Scripts tab or switch costumes.
5. All the components in the costume you previously edited are grouped (error)

Expected behavior

I expect the costume to be grouped exactly as I left it.

Video

https://streamable.com/6kd9h1

Device (please complete the following information):

• OS: Windows 10
• Browser: Firefox
• Version: 83.0