I can't find any evidence that QT/OS X ever supported DTS directly. Maybe you had a QuickTime component lying around that provided support for it, and that was disabled or broke?
I'm in the same boat. You could leave DTS as an audio track, and as long as you had an AAC track to go with it, it was fine - it would play in iTunes, Quick Look, and on AppleTV. Now they are all not working, unless you go in and delete the DTS track. Really, really obnoxious. Why would they do this?