Per title, the '(Annual) Gauntlet Contender' achievement remained at 11/12 after completing the 'Gauntlet Contender' achievement.
Gauntlet contender carries over from year to year, so if you already defeated e.g. 1 last year and then killed the other 11 this year, you'd be 12/12 on the perennial achievement and still 11/12 on annual.
Hype is the path to the dark side. Hype leads to unfulfilled expectations. Disappointment leads to anger. Anger leads to disgust. Disgust leads to "oh, new shinies! I'm back!"