Sometimes there is a hazard during enabling ACK. Library sends <enable xmlns='urn:xmpp:sm:3'/> and immediately sends other stanzas (<presence/>). Very often, on first ACK response, received and expected h parameter are differ by one.

It seems, the only solution (contrary to [XEP|https://xmpp.org/extensions/xep-0198.html#acking]) is waiting for <enabled xmlns='urn:xmpp:sm:3'/> (or error), before sending everything else.