I’d love an answer to this as well. I started my account after Christmas, then played through all the Wintersday Tixx events, and the jumping puzzle. I don’t see any chest next to Tixx, nor did I get anything in mail.
If anyone else could post whether they started doing events after Christmas and got the chest reward, that’d be great. I don’t want to file a bug report if that’s the way it’s supposed to work.