Disgruntled users have been reporting a major issue with Assassin's Creed III's first proper DLC, The Hidden Secrets pack, which was made available to Season Pass owners yesterday. Apparently playing the content is causing players' progress through the main story to be reset, while preserving progress on certain side missions and activities.
A 100% completion statistic is therefore being rolled back to 45%, placing players at the game's opening chapters, meaning essentially having to begin the story again from scratch. In some cases, completed Naval and Privateer missions have also been reset too.
As Assassin's Creed III autosaves, it seems that completed saved games might be unobtainable. It would be advisable therefore to avoid playing The Hidden Secrets DLC to prevent your main mission progress from being erased. We'll update this post as we hear more about it.
You can find out more in Ubisoft's official thread or in our own Assassin's Creed III forum thread on the subject.
Source - X360A