How to reproduce:
1. Open the attached “IN-150678.zip” Project
2. Run the Mesh Serialization Script in the Editor (Tools → IN-150678 → Create GrilleY.asset)
3. Open the newly created “GrilleY.asset” file in a Text Editor or IDE
4. Observe the m_IndexBuffer value and its final character
Actual result: A stray non-hex character is appended “y”
Expected result: No additional characters are added to the value
Reproducible with: 6000.6.0a7, 6000.6.2f1, 6000.7.0b1
Not reproducible with: 6000.0.84f1, 6000.3.24f1, 6000.6.0a6
Reproducible on: macOS 26.6.2 (M3 Pro), macOS 26.5.2 (M5) (user's)
Not reproducible on: No other environment tested
Note: User has noted that the issue might actually be a documentation issue that was not recorded in the Unity Documentation, exact comment:
”After further investigation on my side, I believe this is an intentional format change rather than corruption: the trailing letter is a type tag on hex-serialized arrays (y = UInt8, n = SInt8, s = SInt16, t = UInt16, i = SInt32, u = UInt32, l = SInt64, q = UInt64), new in 6.6, and 6.5 reads it back fine.
the text serialization format changed in 6000.6 and nothing documents it — the YAML format description page does not mention the suffix, and every strict hex/YAML consumer outside Unity (merge tools, validation hooks, asset pipelines) now sees invalid data.”