Stream Management is a feature described in XEP-0198: https://xmpp.org/extensions/xep-0198.html

Stream Management allows for an XMPP session to be resumed after its connection has been (unexpectedly) dropped. To be able to do this, the XMPP session is kept alive for a certain time after the disconnect. If, after this period, the session has not been resumed, its closed (sending 'unavailable' presences to all of the contacts, etc).

There's a bug in Openfire that causes this disconnect to occur on a new XMPP session that has been established after the original session was closed, if that session is established using the exact same full JID.

Openfire apparently:

• sets a 'detached' state for XMPP sessions, even if the session was closed properly.

• identifies sessions to be cleaned up after a detach TTL by JID, not by a more unique identifier.

• does not purge the 'detached' sessions when a new session using the same full JID binds successfully.

Update: it appears that when the first session closes formally/properly (eg: with </stream:stream>, the problem described in this issue is not reproducible.