It appeared with the release of 1.2 patch that achievement could be copied from one user to another by using the Compare Profile feature. That means that a further advanced player could regress by comparing his achievement with those of a less advanced player. This only affects achievement and no other part of player profile.
1.2.1 patch fixes this bug and now prevents achievements to be copied.
When the patch will be live?
Patch will be released on January 5th for all territories.
Did it affect all the players?
All players using the Compare Profile feature upon 1.2 release could get affected. But if you did not use it, then your achievements are safe. Of course, this issue is not happening anymore with 1.2.1 patch.
How can I check out if it has affected my account?
Go to the Achievements menu and check if previously unlocked achievement are now locked again. It worked the other way too, if you have compared with a further advanced player.
After the patch release, will I get back all my lost achievements?
All achievements can be retrieved again but not automatically. Some of them will come back very easily, like XP and Dynasty Level related ones since it will update once you gain level in each area. Unfortunately, other achievements rely on user progress and require to play a bit to get them back. None achievement are lost forever.
Can I get rewards for the achievements twice?
No, achievements unlocked after being lost through Profile Compare will not award the Dynasty Seals again.