Initially, I thought that the issue is on the server-side and was checking server code related to stream resumption, but as I've looked closer the issue appeared to be in the test which "reconnected" almost instantly but in fact, the connection was not fully established as SessionObject kept old bound resource and due to that Jaxmpp was not waiting for proper reconnection.

The issue was fixed by removing bound JID from the SessionObject in the test case simulating reconnection on connection drop.