Poezio should always send @jid in affiliation changes
https://xmpp.org/extensions/xep-0045.html#grantmember
0045 says that @jid should always be included in <item/>. Then if @nick is included, this nick is also registered for the user.
This function from multiuserchat.py seems like a good culprit.
Not sure if that needs a change in the /affiliation command's interface.
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