It’s a typical Christmas disaster, firstly there was the squirrel in the Christmas tree, sorry wrong store. It ’twas the disappointment of players who didn’t get a trophy when they unlocked Christmas Gnome Stewie, now other players still working on unlocking him are seeing materials they’ve collected disappear, whilst they are awarded a quantity of medals instead. I’ve alerted support, here’s hoping we get a Christmas miracle and it’s sorted before the holidays. But for those with the issue know you’re not alone.
The only advice I can give is visiting Ollieland seems to guarantee you’ll lose any items collected so avoid it if you’re planning to try collecting until there’s a fix.