XEP-0359 defines a stable and unique stanza identifier. It suggests ("SHOULD") that the value that is used is a UUID, but does not require so.
Currently, Openfire assumes that XEP-0359 specified stanzas are UUIDs. This need not be the case. Even though Openfire generates (and will continue to do so) values as UUIDs, values that are supplied by third party code needs not be a UUID (converse.js 6.0.1 is known to add a postfix to an otherwise UUID value, making the resulting value no long a valid UUID.