- Play the game logged in into Steam account A;
- Start new game on the same computer logged in into Steam account B to which the game is granted under Steam Family Sharing;
- Expected result: progress, perks and collectibles are separate for Steam account B, they are also preserved for Steam account A;
- Actual result:
- Progress is reset for account A;
- Perks are reset for account A;
- Collectibles are *not* reset for account B;
- Steam account B gets stats and ranking where existing stats and ranking for account A is a base to which personal stats of account B are added;
- Steam account B gets *some* achievements based on above stats calculation.
Fallout for Steam account A owner: [at least] can't selectively play game chapters even though the game was already completed.
Lesson: different players must play on different computers, or get really sophisticated about backing up game data, taking risks of likewise sophisticated screwups.