When the results returned by the server are complete (that is: when they have not been limited by the maximum size of the result page (either as specified or enforced by the server)), the server MUST include a 'complete' attribute on the element, with a value of 'true'; this informs the client that it doesn't need to perform further paging to retreive the requested data. If it is not the last page of the result set, the server MUST either omit the 'complete' attribute, or give it a value of 'false'.
OpenFire lacks it on their MUC:
openfire@conference.igniterealtime.org
and Siskin is getting stuck querying MAM constantly:https://xmpp.org/extensions/xep-0313.html#query-paging-request
Ref: https://issues.igniterealtime.org/browse/OF-2063