I had a spayed GSD (spayed before her first season) who would mother anything, she adored pups.
At the same time I had a BSD who was entire. I never bred from her - BUT I always said that if I had then my GSD would have to take the pups and rear them because Lucy (the BSD) did not have a maternal bone in her body!!!
I don't think spaying has anything to do with it.
Sue