On Friday we pushed a bug fix that created another bug: tickets were sorted by the date they were created, not by the last reply. This was unintentional and obviously very confusing (as it wasn't clear that someone had recently responded to a ticket). That said, it did not affect the Open status of unread tickets, so you shouldn't have lost track of any tickets. We discovered and fixed the bug this morning. All tickets should now be sorted as usual – by last reply.
We apologize for the issue and the confusion – thanks to those who reported it!
Community Manager, UserVoice