Hello David
The behavior that you describe in your post is normal. This is because the auto-summary feature is disabled by default on all Cisco IOS devices, as confirmed in this command reference. documentation.
Without the auto-summary feature, the prefix and mask used in the network command must match exactly a prefix with the same subnet mask within the routing table. Since your command network 11.0.0.0 has no mask keyword or argument, the classful mask of 255.0.0.0 is used. However, there is no 11.0.0.0/8 prefix in the routing table. (There is however a 11.0.0.0/24 prefix in the routing table).
Once you enable auto-summary, BGP will automatically advertise the classful network if you have the classful network or a subnet of this network in your routing table. In your case you have a subnet of this network, thus you see the output in the show ip bgp command.
I hope this has been helpful!
Laz