Spark is not trying to automatically reconnect after the server is shut down or connection was lost. It shows either "connection lost due error" or "server was shutdown", but not starting a count down to reconnect.
Smack 4 introduced a ReconnectionManager. When migrating from 3 to 4, I disabled the old reconnection mechanism (which is no longer available in 4), but neglected to enable the new mechanism. That's now fixed.