Hello,
This is normal, if we add the Class A, Class B & Class C network without sub-netting the display current-configuration shows the result without subnet-mask.
Below are the test result, Kindly confirm if any other quires about the issue.
Adding Class A network :

Result :

Similarly, if we add Class B & Class C network without sub-netting

Result :

But when we add the networks with sub-netting the result will show with a subnet mask.

Result :
