Trying to create a slice animation

Hi, I’m currently trying to recreate a material that acts as a slice plane (ideally to then use for animating the slice to have objects show up) but the source material is 7 years old and pretty bare bones. Here is the link to it: ArtStation - Slice / Cut Material

I managed to create the material and the slice works, the issue I’m having is that my object has a reflection/mirror effect where the slice is, or the object becomes hollow after it’s sliced. In the source video the object stays solid in the middle as it’s sliced.

Any idea on how I can fix my slice material so that when i apply it to an object and use the slice offset my object stays solid? Or a better way in general to get a slice animation done in Unreal Engine.

Thanks!