The ChainGeometry type has a minimum limit of 4 vertices when being created. This stops it being used to create a PhysicsChain even when used with a PhysicsChainDefinition.isLoop as true.
In this mode, no ghost vertices should be present in the ChainGeometry as ghost vertices are automatically created using the start/end vertex in the geometry.
Discussions: https://discussions.unity.com/t/allow-chaingeometry-to-have-3-vertices/1715149