DNS Stub Zone - Problem with one hostname resolution

Dear all

I have a strange(?) problem with a DNS Stubzone.

My Setup:

My Domain: a.bla.com (Win2k8R2)

Other Domain: b.bla.com (Win2k12R2)
file.b.bla.com - 192.168.2.50
www.b.bla.com - 192.168.2.51

I created in the DNS Servers of both Domains a DNS Stub Zone to the other Domain.

Now i am sitting in Domain a.bla.com:

nslookup file.b.bla.com --> Works, i get 192.168.2.50 as answer
nslookup www.b.bla.com --> can't find www.b.bla.com: Non-existent domain

If i do nslookup www.b.bla.com in the b.bla.com Domain it works as it should.

I dont understand what is wrong and why it is not working with www.b.bla.com.

Thanks for any hints.

JBAB