Another way to design this, if it's something you want as a baseline (that a child does not follow the parent), just structure the relationship like this
Parent:Spatial > Containerchild:Node > desiredgrandChild:spatial
what happens is that because standard Nodes don't have transform data, they don't pass on the transform information of the parent to the grandchild, meaning that you now have a way to create independent rotation of a parent from a grandchild
However, this also means that if for any reason you do need to rotate a grand_child with the parent, it will have to be done through code somehow... (maybe by using re-parenting?)
I learned of this because if you have a viewport as part of a scene, then the camera childed to the viewport will not move along with the scene root, because viewports do not have transform data.