Camera with an "Uninitialized" Background Type, higher Rendering Priority and a custom Viewport Rect does not show lower Priority Camera layers when having multiple Cameras in the Scene

Uninitialized does not meant "don't clear". On some platforms, the uninitialized color buffer will contain data from the previous frame. On other platforms, the uninitialized color buffer will contain uninitialized memory. To achieve the expected result, you'll need to use the main camera as "Base" and the other cameras should be set to "Overlay". See https://docs.unity3d.com/Packages/com.unity.render-pipelines.universal@17.0/manual/cameras-advanced.html for more details.

Cancelling this 2023.2 tech release issue since the support for this release has come to end of life. Cancelling this 2023.2 port case does not affect any LTS or Unity 6 Beta/Preview that remain open.