Let's imagine that your "original" Z, A and B courses are in server 1. Z is the "owner" of the questions.
If you backup and restore A or B in server 1, then the new courses, A2 or B2, will use Z questions too.
If you restore A or B in another server called 2, if Z exists there, the new restored courses will use its questions too.
But if Z doesn't exists in server 2, A or B will restore the questions as owners. This is the expected behaviour to ensure the integrity of the quizzes across different servers !!
So, I recommend you that always restore your Z (repository) course in the new server before restore the A or B (dependent) courses on it.
Has this sense for you ?
Ciao,