Slices are sometimes linked to frames and sometimes aren't

Reproduction steps:

1. Start with an empty file with 20 frames
2. Create a slice on frame 1.
3. Move to frame 10 and change the slice's size. The editor now shows the slice at one size for frames 1-9, and a different size for frames 10 onwards.
4. Move to frame 5 and add insert some new frames

Results: the slice edits don't move with the frame where they were made, they stay on frame 10. That is, whatever assets were in frames 8-9 now have the wrong slice.

Aseprite and System version

• Aseprite version: Mac 1.2.16.3