Reproduction steps:
1. Open project “Repro.zip”
2. Open “Sample” scene
3. Select the “Player” GameObject in the Hierarchy
4. Expand the "ModifyBoneTest" component in the Inspector, and make sure the value of the "Single Layer Optimization" property is set to false
5. Enter Play Mode
6. Drag the "Bone Alpha" slider to change the value of the "Bone Alpha" property in the Inspector
7. Observe the pose of the player's head in the Scene view

Expected result: The pose of the player's head *changes*
Actual result: The pose of the player's head *does not change*

Reproducible with: 2021.3.28f1, 2022.3.4f1, 2023.1.4f1, 2023.20a23

Reproducible on: macOS Ventura 13.2.1 (Intel), Windows 10 (by the reporter)

Notes:
- Also reproducible in Standalone Player
- Enabling the "Single Layer Optimization" property in Editor mode and then entering Play Mode will give expected results, but modifying it during Play mode will not take effect
- Setting the “Bone Alpha” property to 0 then entering Play Mode and barely sliding the value makes the head rotate and then break again