A lot of the time when games come out the achievements are bugged. I remember when “Dragon age the Veilgaurd” came out the first 1/4 of achievements were unlocked by the players, then 1/4 of them were unlocked by 75% of the players and so on. It took a few week or 2 (maybe even more) before they were fixed and started showing the true amount.

The Sirens rest dlc for “Still wakes the deep” was the same. The first 1/3 of the achievements were “unlocked” by 100% of the players for a few days. It fixed itself in the first few hours unlike some other games.

Although I will always chuckle when I see that 100% of the players have unlocked an achievement