It's supposed to be like that. The network map service is not a presence service in the sense that an instant messaging server would be. Entries in the network map never expire in M13. We are currently doing some work to redesign the network map service to change how it works, to make it a static signed file available over HTTP CDNs for robustness and scalability. As part of that we're introducing the concept of an event horizon ... the amount of time you can be offline until nodes are allowed to forget about you. It'll probably be quite long, like 30 days.
The idea is that Corda networks can tolerate temporary outages of nodes. You can still start flows with them and the flows will wait until the node comes back.