How to reproduce:
1. Open the attached “Viewer” project (in “IN-124519 2.zip”) in 2022.3.69f1 version
2. Open “SampleScene“
3. Enter Play mode
4. Observe the Console window. Notice the 2 warning messages: “Some script has started” and “Some other script has started”
5. Close the project
6. In Unity hub, upgrade the “Viewer” project to any Unity 6 version
7. Open the “Viewer” project
8. Open “SampleScene“
9. Enter Play mode
10. Observe the Console window

Expected result: The custom warning messages appear: “Some script has started” and “Some other script has started”
Actual result: The Unity Editor warning messages appear: “The referenced script (Unknown) on this Behaviour is missing!”

Reproducible with: 2023.2.0a19, 6000.0.64f1, 6000.3.1f1, 6000.4.0b1, 6000.5.0a3
Not reproducible with: 2023.2.0a16
Could not test with: 2023.2.0a17, 2023.2.0a18 (Crash on _platform_memmove when opening the project)

Reproducible on: macOS 15.3.2 (24D81) (M1 Max)
Not reproducible on: No other environments tested

Notes:

• Removing the “Library” folder after updating the project does not resolve the issue
• The bug is not reproducible with the following local package paths:

{{"file:../../aViewerShared/com.revizto.revizto-shared"}}

{{"file:../../Unity6000-ViewerShared/com.revizto.revizto-shared"}}

{{"file:../../SharedPackages/com.revizto.revizto-shared"}}

{{"file:../../SharedPackages/ViewerShared/com.revizto.revizto-shared"}}

• And reproducible with these local package paths:

{{"file:../../ViewerPackages/com.revizto.revizto-shared"}}

{{"file:../../ViewerShared/com.revizto.revizto-shared"}}