I tried uploading some meshes today, their poly count was higher than 5k so they were rejected by studio.
Even though they were rejected in studio they still uploaded to the website, the mesh is unusable.
When I insert this mesh from the toolbox it inserts a MeshPart with size 0, 0, 0.
I dont think this was intended 
Can you send me a copy of that mesh?
I unfortunately don’t have it anymore.
But in my case it did this for any mesh with too many polygons.