Reproduction steps:

1. Open the attached project “ReproProject“
2. Open the scene “Scene“
3. In the Hierarchy Window, select any of the “Trail“ GameObjects
4. Observe the result

Expected result: ParticleSystemVertexStream.PercentageAlongTrail returns data percentage strictly ranging from 0 to 1, therefore the color is white-black gradient is applied for all trails present in the scene
Actual result: Data is not always returned in a range of 0 to 1 for all “Trail“ GameObjects, only the trail with Texture mode Stretch“ returns the correct data. Where incorrect data is present, the red or blue color assigned instead

Reproducible in: 2022.3.56f1, 6000.0.33f1, 6000.1.0b1
Could not test with: 2021.3.47f1 (Material which is used for trails to apply color is black for unknown reason, this colors all trails in black)

Reproducible on: MacOS 15.2 (M1 Pro), Windows 11 (user)
Not reproducible on: No other environments tested

Notes:

* Also reproduces in Player
* Possibly, the trail with Texture Mode set to “DistributionPerSegment“ also returns the data correctly, but it seems that the value is fluctuating and the “flickering“ effect can be observed