yes,
FGDATA next submodules follows the development branch which is a carbon copy of the development branch in Thorsten git repository.
The master branch is a multiple merge. And follows not only FGAddon but other stuff too. it mainly focuses on Thorsten git repository master branch (as in merging with that as priority).
Since it is a complex situation, thus FGDATA next (with submodules) is and should follow the actual development fork of the aircraft. Thus, I can't understand why are you in master on the SpaceShuttle. If you do git submodule update --init it should be the correct branch instead.
Part of the confusion arises by how Thorsten treats the master branch in his repository. Which I definitely don't care deeply about as soon as I am followin' the proper branch -- and unless you are going to start developing a fork of the SpaceShuttle, you kinda should follow this way.
Feel free to ask for clarifications if needed.