Original bug report: https://bugzilla.gnome.org/show_bug.cgi?id=660973
See https://bugzilla.gnome.org/attachment.cgi?id=203436 for logs.
Looks like Gabble switched to user's PEP nick when he received a PEP update. It should continue to use the one stored in the roster as pet name should always have the highest priority.
Any progress on this? It is starting to annoy the hell out of me! :)