There is a significant difference between the World Cup adventure buffs and the Xmas adventure buffs. The World Cup buffs were made from collectibles specific to the event that were gained through multiple methods including collectible searches and explorer searches. The collectibles used are event specific and have no use in regular play, similar to the candy canes, bells, and gingerbread. In the past, the collectibles were simply removed from the visible storage but held for the individual player; the current system would remove them completely at the end of the event. Allowing a refund of those buffs would return them to their component collectibles only. The Xmas buffs are made from game resources that are used throughout the game, and therefore the conversion back to game resources was put into place.
The continued game presence of the World Cup buffs and collectibles was likely an oversight with the removal process now incorporated into each event. Now that the problem has been brought to public attension, the developers should be able to address it. Hopefully we can get some information about what will be done regarding them once the decision on how to address this has been made.