Hey @SophiaWolfie, how are you? Hope you’re doing great! 
How are you applying the mask cut effect on the cube? Is the Post Process using the scene Depth information or a Custom Stencil Value?
You should be able to tell the renderer to draw the Depth and Stencil of the cutting geometry (in your case, the plane) without rendering it in the Main Pass. Check the Rendering options of the StaticMeshComponent for the plane, look for “Render In Main Pass”, and disable it.
To get the effect working correctly, you might need to:
Make sure the geometry used as a mask only populates the Stencil Buffer (or a Custom Depth instead of the default scene depth).
Use that Custom Depth or Custom Stencil Value in your Post Process Material.
Ensure all “mask” geometries don’t render in the Main Pass.
This video will helps! https://www.youtube.com/watch?v=JH07z9Ap1hk
Hope this helps!