Not sure if this is an actual bug or if the comment needs to be updated, but TransformNode.rotationQuaternion does not default to null like the comment says.
The comment for TransformNode.rotationQuaternion says:
Gets or sets the rotation Quaternion property : this a Quaternion object defining the node rotation by using a unit quaternion (null by default).
If set, only the rotationQuaternion is then used to compute the node rotation (ie. node.rotation will be ignored)
In this example it is not null and is actually undefined: Babylon.js Playground
I am interested in getting started with contributing to Babylon, so if it’s a bug I would be willing to fix the issue to get my feet wet with contributing.