To reproduce:
1. Open a new project
2. Import any image into the Assets folder
3. Click on the image and in the Import settings, select "Sprite" and "Multiple"
4. Click on the "Sprite Editor" button
5. Dock the sprite editor
6. Manually create a slice.
7. In the bottom right box, highlight the filename
8. Manually create another slice
9. Notice that the filename did not get updated
10. Click anywhere so the sprite editor loses focus
11. Notice that the filename is updated