OK, that’s consistent. The SmartThings hub as a secondary controller does not query the primary for network updates. So the only devices it will know about is the ones which were already connected to the primary at the time that the hub joined the network. If you want to add new Z wave devices after that time, you have to remove the smartthings hub and then re-add it each time after you have added new Z wave devices.
This is one of the main reasons they officially “strongly advise against” trying to use it as a secondary. It’s technically possible, but very primitive.