This started happening right after updating studio & the player, just now.
I am not 100% sure what causes this, if it is cloning MeshParts or destroying them. We are using ViewportFrame objects in our game and putting MeshParts in them. These had some major LOD issues before, but now those issues are gone. Instead, there seems to be a chance for studio/in-game to crash completely.
Here is a game where it happens (private):
If any staff goes in game, please press C and then X to refresh the ViewportFrames.
After one or more times, it will cause the game to crash.