Mostly, that's because getChanges in PhabricatorCalendarEventInviteTransaction is wrong - the phids in this transaction are in keys, not values.

But more disturbingly, there's some bug with the rendering too:

As it is, the text renders like this;

but the string used in pht is %s invited %s attendee(s): %s; uninvinted %s attendee(s): %s..
Adding a space before the first %s renders ok:

Not a rendering bug, it's a translation: https://secure.phabricator.com/diffusion/P/browse/master/src/infrastructure/internationalization/translation/PhabricatorUSEnglishTranslation.php$1571

Maybe I shouldn't debug things before coffee. getChanges is fine, the problem is something else; I think the recorded information might be wrong.

Thanks :).