To reproduce:
1. Open my attached project
2. Witness two sets of sprites - both have one sliced and one simple sprite
3. Optional: try RectTransforming - sliced sprites seem to be functioning normally

Expected: sliced sprites should be able to maintain original aspect as well
Actual: sliced sprites get stretched (check .gif attached for results)

Reproduced in: 5.5.5p2, 5.6.4p4, 2017.1.2p4, 2017.2.0p4, 2017.3.0f1, 2018.1.0a7
Not reproduced in:

Note: preserving the aspect selection would be great here, such as the one in Simple image type parameters. As currently making two identical buttons is tedious
Note: it's possible to some extent to try matching sprites by changing Pixels Per Unit field