How to reproduce:

1. Open any (new) project
2. Import any image
3. Make sure it is imported as a sprite
4. Set the sprite mode to Multiple
5. Set the image Max Size to be smaller than the actual image size
6. Apply Import Settings
7. Open Sprite Editor
8. Slice the sprite in a grid using a larger number than the Max Size of the image
- Notice the slice size is set to the Max Size of the image
- Actually slicing it does not create one slice of the image, but multiple slices because the slicing pixels appear to be counted as the actual pixels of the image
- Can be worked around by simply having Max Size be larger than the image size