I only have one server that I can test this on, so I'm not entirely sure if it is a bug in detecting IP addresses or more of a bug in the UI. I have a server with a single network card (eth1) with multiple external IP addresses. If I go to Webmin > Networking > Network Configuration > Network Interfaces, it correctly lists eth1 several times with different IP addresses. However if I go to Virtualmin > Server Configuration > Change IP Address, there is a select box under "Shared address" with only one option in it, the main server IP. I believe that the other IP addresses should show up in this list, but they do not. If this is by design, there should be some way to easily select the actual IP addresses available on the server as listed in /etc/network/interfaces. In the majority of cases, it should not require the user to manually enter the IP address.
That is by design - if you want to use other IPs on your system as shared addresses (which can then have domains changed to use them), they must first be registered at Addresses and Networking -> Shared IP Addresses.
Should not the other IP addresses be recognized upon Virtualmin installation then?
Not necessarily ... not all the IPs on your system may be for use by Virtualmin domains.