Stream management or SM is a feature designed for mobile world. It keeps client session online when a client is disconnected for a short time. The benefits of doing this: a) client doesn’t have to go through the process of establishing a new session, which is a lengthy process, so it can get online faster after the network is up again, b) psycological effect - if you go online and offline every few seconds because of a flaky mobile internet, the others would hesitate talking to you fearing that you wouldn’t get a message. With SM you appear online and everything seems fine.
That’s at least how i understand this feature. There is another setting xmpp.session.detach.timeout which controls SM timeout, which is by default 10 minutes (setting in milliseconds). You can either try lowering it or disabling SM with the setting you have mentioned, if SM is of no value to you.
Recent ticket https://issues.igniterealtime.org/projects/OF/issues/OF-1813