This has been asked for a few times, unfortunately there’s no nice / self-contained way of displaying 3D Elements in the HUD atm. I guess you followed This Tutorial to do the above, but unfortunately the technique has nasty edge artefacts.
Since you’re using FPreviewScene, can you render the mesh in CustomDepth and mask out based on that instead? I’m not sure how or if FPreviewScene supports it but it probably does. Might be a good temporary workaround until we have 3D Mesh support in the UI.
FYI the reason this isn’t a feature yet is because you need different versions of the shaders for all the meshes… or something like that. They don’t have a nice way of doing that atm.