It only looks transparent in certain lighting conditions. The shadow receiver material is a normal opaque material, but the background is an unlit material.
You need to tweak the emissive color value in the BaseFeedScreen material (opaque shadow receiver material) to look identical with BaseFeedScreenNoShadow (unlit video background material).
There is a division node between the color and the emissive parameters. Try to tweak that parameter to make it match with your light setup.
Divide with:
- Higher number -> darker look
- Lower -> lighter look
For the tweaking make the shadow receiver plane smaller so you will see it’s borders better from any angles.