Summary
While using AR Simulation to navigate in Play mode in the PlaneOcclusion Samples scene, the walls occlude the checkered cubes as expected. However, the detected plane for the image marker cube is larger than its actual geometry. See the cubes indicated by red arrows in OversizePlane.png attached.

To See the Bug
Open the ARF Samples project in the Editor.
Open the PlaneOcclusion scene.
In Build settings, select desktop platforms.
Enable XR Simulation in Project settings.
In Scene view, use the AR Simulation overlay to select DefaultSimulationEnvironment.
Enter Play mode.
This scene includes a cube with the Simulation Image Marker component; it is scaled to look more like a quad.
Navigate in Game view so that the Image Marker cube is in view.
Click on the Image Marker cube.
This generates checkered cubes.
Navigate so that the edges of the Image Marker cube occlude some of the checkered cubes. The occlusion region extends beyond the Image Marker cube in some places. This is the bug.

Configuration
2021.3.0f1.2 (2021.3/staging 6eacc8284459)
MacOS 12.0.1, MacBook Pro (15-inch, 2019), 2.4 GHz 8-Core Intel Core i9
AR Foundation 5.0.0-pre.10 (f6569c5)