How to Reproduce:
1. Open the attached “IN-145198” project
2. Open “SampleScene”
3. In the Hierarchy, select the “Timeline” GameObject
4. In Timeline window at frame 5500 observe the "UP" and "DOWN" clip's order
5. In Timeline window on Custom Track at frame 6000, right click and choose "Add Custom Track Clip"
6. Observe the clips at frame 5500
Actual Result: Order of "UP" and "DOWN" clip has interchanged
Expected Result: Adding "Custom Track Clip" does not change other clip's order
Reproducible with: 1.8.6 (6000.0.0f1), 1.8.12 (6000.5.1f1), 6000.6.0b1, 6000.7.0a1
Reproducible on: Windows 11
Not reproducible on: No other environments tested
Note: Not only these clips but some other clips change order as well.