How to reproduce:
1. Open VM -> Bug Attachments -> '1202611-postprocessing-order-issue.zip' project (more information in Edit)
2. Navigate Edit -> Project Settings -> HDRP Default Settings -> Custom Post Process Orders
3. Reorder Post Process Orders in Before Opaque And Sky panel
4. Restart the Editor and repeat the second step

Expected result: Custom Post Process Orders are reordered and saved when Editor is restarted
Actual result: Custom Post Process Orders are reverted when Editor is restarted

Reproducible with: 2019.3.0f3 (7.1.6), 2020.1.0a16(7.1.1, 7.1.5, 7.1.6)
Could not test: 2017.4.35f1 HDRP is not introduced, 2018.4.14f1, 2019.2.16f1 HDRP Default Settings are not introduced

Notes:
1. Video of the reproduction attached in Edit
2. Reproducible with Before transparent(7.1.1, 7.1.5), After Opaque And Sky, Before Post Process, After Post Process Orders
3. A workaround for the issue is to remove the effect from the list and re-add, then reordering is saved
4. Not only a visual issue, functionality is also affected