When a repository containing submodules is imported as a submodule, only the first submodule is imported, and it is not possible to build with the second stage.
The build itself is successful, but not intended to be.
This was the same for both PUBLIC repo and PRIVATE repo.
The command for cloning from Github is git clone --recursive.
You’ll need to do something different there, like clone your own sub-submodules before building, which is pretty complicated unless they are public repos. See this article for details:
I’ve added your voice to our open feature request on better submodule clones, so I’ll follow up in this thread in case we change that behavior. I do not expect a change anytime soon though, so if you want to use our service, probably best to find a workaround.