To celebrate the release of Halo 3 on PC, I played through the campaign yesterday, progressing through and completing challenges in the process. I made good progress on the Seasonal and Celebration Challenges, however I logged in today and noticed something strange with those challenges. Any complete challenge remained complete, but a few incomplete ones (not all of them), like Acrophobia, Plasma Master II, Hot Shotty, and Corps Values I & II had their progress reset. I tried playing a mission and booting the game back up to see if this had anything to do with logging in, but it appears that is not the case.
Yup, looks like this is a problem for everyone. This feature should have been ‘flighted’ before being put in the release.
For me as long as I don’t switch games then it ends up working for me even if I restart the game. Hope that helps 