Some steps I have attempted to resolve the issue is turning off StreamingEnabled and removing the collision groups on the vehicles, however neither of these had any effect.
I’m not sure if this means anything, but in your test place, when I set the CollisionFidelity of the Union to PreciseConvexDecomposition the car no longer falls through the map. You could try playing with those settings to see if this provides a temporary fix for the issue.
Hence why I said to play around with the settings to see if any worked best, though I was testing unions with the different fidelities and didn’t seem to have any issues with them. That post also only mentions MeshParts, not Unions, that said, my game is made almost completely out of MeshParts and I’m not having any collision issues.
