The repro project uses two base cameras, one in each additive scene. One base camera has set it's background to uninitialized. With Compatibility Mode enabled, the unitialized background of this camera happens to be what the first camera drew previously, which is why both camera's output is visible. This is incidental behavior, that should not be relied upon. "Uninitialized" means the frame buffer can contain anything. When trying to render two cameras on top of each other, setting the top camera to "Uninitialized" is not the correct approach. Instead, camera stacking should be used, and the top camera should be configured as an overlay camera. Please refer to https://docs.unity3d.com/6000.0/Documentation/Manual/urp/cameras/camera-stacking-concepts.html for more information