Until support get back to me after the weekend, the only advice I can give you is don’t visit Ollieland if you’re planning to try collecting the materials again for Christmas Gnome Stewie. Visiting Ollieland seems to be the cause of you loosing any items you’ve collected so avoid it until this is fixed.
UPDATE – SUPPORT ARE AWARDING PLAYERS AFFECTED CHRUSTMAS GNOME STEWIE AND THEIR TROPHY, ALLS WELL THAT EBDS WELL.