Reproduction steps:
1. Open the attached project "ReproProj"
2. Open the “/Assets/Scenes/SampleScene.unity” Scene
3. In Hierarchy select “Issue Canvas“
4. In Inspector, change “Additional Shader Channels“ to “Nothing“
5. At the top of Inspector, click “Overrides” drop-down click “Canvas“ and click “Apply > Apply to Prefab 'beginScreen'“
6. Observe “Additional Shader Channels“

Expected result: Value stays at “Nothing“
Actual result: Value gets reset

Reproducible with: 6000.0.68f1, 6000.3.11f1, 6000.4.0f1, 6000.5.0a7

Testing environment: Windows 10 Enterprise 21H2
Not reproducible on: No other environment tested