ALL SEASONAL CHAMPIONSHIP ACHIEVEMENTS ARE BUGGY-!
I think I found the best way to avoid the bug and unlock all seasonal championship achievements on first try! 4 of 4 for me in a row! Before this method I just played randomly, then got the bug, no cheevos for me.
Please vote positive or negative if you tried this and worked or not worked!
1. Play in Horizon Solo (Pause menu / Horizon Life tab / bottom right corner)
2. Play against drivatars in Solo
3. Choose the exact drivatar difficulty you see in the requirements
4. Get 1st place on every race for the first time. You can restart the event any time, just go to the pause menu while in a race.
5. Beat all 3 championships in one sitting (maybe you can put your console into Instant-On mode, but I never tried)
6. Play only the actual season, so do not try to change the system date to get them earlier and make sure you are connecting to Xbox Live