Reproduction steps:
1. Download and extract the project in "case_1316556-UnityBugHunt.zip"
2. Download P4V client from https://www.perforce.com/downloads/helix-visual-client-p4v
3. Launch P4V client, switch to Remote Server tab (if you are connecting to localhost, you will need to launch a helix core (instructions below))
4. Create a new User
5. Create a new Workspace and set the UnityBugHunt project root as the location
6. In the server Field enter "localhost:1666"
7. Click ok
8. Open the UnityBugHunt project using Unity editor
9. Go Edit -> Project Settings -> Version Control
10. Set mode to Perforce
11. Enter Username, Password, Workspace that you created in the P4V client
12. Set server to "localhost:1666"
13. Click on Connect
14. Submit all changes to the Perforce server
15. In Unity go Test -> Test 3 - Asset Reordering Test
16. In P4V hit refresh
17. in P4V Workspace navigate to Assets/Test
18. Right-click on SubAssetTest.Asset and select "Diff Against Have Revision"

Expected result: The order of the assets is the same
Actual result: The order of the assets is different

Reproducible with: 2019.4.26f1, 2020.3.7f1, 2021.1.6f1, 2021.2.0a15
Could not test with 2018.4.34f1 (packages break, package manager not available)

Notes:
1. I put instructions on how to launch helix core below in edits
2. The customer described the issue in greater detail in "Sub Asset have non-deterministic serialization.txt" located in Assets/Readmes or attached below
3. Image with Diff Against Have Revision view is attached below