How to reproduce:
1. Open the user-submitted project
2. Open the Project Settings window and navigate to Player settings (Edit > Project Settings... > Player)
3. Expand the Resolution and Presentation tab
4. In the Resolution Options, set the Fullscreen Mode to "Windowed"
5. In the Standalone Player Options, enable "Resizable Window"
6. Build and run the project (File > Build and Run)
7. Resize the Player window by grabbing the left side and dragging it to the right

Expected result: The letterbox background remains black
Actual result: The letterbox background changes to a light blue color

Reproducible with: 2019.4.31f1, 2020.3.20f1, 2021.1.25f1, 2021.2.0b16, 2022.1.0a12

Notes:
The color was reported to change to red, but was observed to be light blue during testing
Reproducible on Metal and OpenGLCore