This only effects the costume bought with the crowns, and I am not sure if it has been reported or not, so here it goes.
Whenever I log in or zone in while wearing a costume bought from the crown store, the costume will disappear whenever I place a ground effect spell, (playing a Breton Sorc.) ie: Volcanic Rune, Liquid Lightning. The UI however shows that I am still wearing the chosen costume. At this time it is a simple matter of deactivating the costume and reactivating it for its look to return.