How to reproduce:
1. Create a new HDRP template project
2. Create two cubes in the Scene which overlap each other
3. Create two separate material assets and assign to cubes
4. Set different Base Map colors and Surface Type - 'Transparent' in both materials
5. Watch the Game tab and select one of the materials
6. Change Surface Options -> Sorting Priority to '1'
7. Change Sorting Priority to '-1'

Expected result: Cube with a higher priority is displayed in front of another with a lower Priority
Actual result: Sorting Priority has no effect

Reproducible: 2019.1.6f1 (5.7.2)
Not reproducible: 2018.4.4f1 (4.1.0), 2019.1.6f1 (5.16.1), 2019.2.0b9 (6.7.1), 2019.3.0a1 (5.2.3), 2019.3.0a9 (6.7.1)
Can't reproduce:
- 2017.4, because HDRP was not introduced