The issue is that a SceneCapture2D component applies whatever it captures to its texture, but all meshes become transparent space, as demonstrated below.
However, the issue does not apply when this texture is used on a material, as shown.
I have tried making static textures, and there is no difference. I need to use a scene capture texture in a widget image component, but as shown below, it follows the texture rather than the mesh. (Top-right)
EDIT: I figured out that you can use a material in place of an image in a widget, but the transparency in the textures still seems like a bug that could cause problems elsewhere.