@baraterek That's mixing up two things here. For several games, including Anno 1800, all challenges and rewards were completed/unlocked last year with the switch from Uplay to Ubisoft Connect. This was intended behaviour and the Reddit thread you linked is showing those challenges (even if talking about achievements).
The issue from the beginning of September was a different matter, which unlocked the achievements for some players. That was of course not intended and we're currently investigating if and how we can resolve this.