To reproduce:
1. Open attached project
2. Open SampleScene
3. Select TestScript in the hierarchy
4. Notice that size=1024, padding=2
5. Press Play (You can see only the red rectangle)
6. Stop scene and change padding of TestScript to 0
7. Press Play. You can see distorted rectangles.
8. Stop scene. Then change the padding of TestScript to 16
9. You can see correctly combined texture which contains square red, green, blue rectangles.

Actual result: padding =2 doesn't work correctly when textures are compressed with RGB Compressed ETC 4 bits format.
Expected result: padding =2 should work correctly when textures are compressed with RGB Compressed ETC 4 bits format.

Reproducible with: 2018.1.0a5, 2018.4.18f1, 2019.3.4f1, 2020.1.0a25, 2020.2.0a1
Not reproducible: 2017.4.37f1, 2018.1.0a4

Note: This issue happens only if textures are compressed with RGB Compressed ETC 4 bits format.
When setting a texture size to 512, padding = 2 works fine.