Patch 1.2.1.0 introduced achievements, but not all of them were unlocked for me, for instance The Attunement one, Beast, Hope, Teacher, Watcher, Falsehood, and possibly some of the secret ones as well. I know I unlocked a bunch of them on my first playthrough, but they don't really show up anymore.

Yeah I've got the same problem. I've got most of the achievements including the dream nail and awaken all the beasts but
not a single one of them I got after the update.
Also the amount of hours I played isn't displayed (but that isn't a real problem)

I have the same problem. I updated the game through GOG Galaxy, and after the update all my achievements were gone. Some of them came back after some playing, but others didn't.
Someone has opened a topic on reddit about this : /r/HollowKnight/comments/793fvc/grimm_troupe_update_took_my_achievements/

My achievements are messed up too. Some of them came back, but others didn't. If they don't fix it, I might have to play it over again (which wouldn't be that bad, actually.

None of the Legendary tier achievements were unlocked. I had Over 1450 essence, but the achievement for 600 essence was still grayed out, until I slapped a ghost with the dream nail, that gave me 1 essence, and the game finally figured out I was over 600 and gave me that achievement. But i don't know how that can be fixed with the Dreamers. 92% game completion rate but only 35% achievements? I find the difference a bit odd.

Yeah I've got the same thing with the essence. Now I got the achievement only of 600 essence but not awaken the dream nail. Pretty frustrating.

Same issues, plus the loading times for random areas are really long now. Namely around the town.

Same here.Update broke achievements which were already unlocked.
Edit: Just downgraded through Galaxy and at least the in game achievements returned.

I have the exact same problem, as soon as I downloaded the update, all of my achievements were gone. Some came back when I reloaded my save (just before the final boss). I know achievements aren't that important but when you did your best to do a game thoroughly to have a 100% and have it all gone is a bit... upsetting.

I think this is fixed now with patch 1.2.2.1, just fire up the save slot and presto, achievements will pop.

Edit: But I still don't think it's all of them. I'm pretty sure I unlockd the Hope achievement in-game some while back, and possibly a couple of other secret ones.

I had the same problem (Hope not popping). All you have to do to reclaim this achievement is traveling to the secret stag station again.

As happy as I am that they fixed the Achievements it would have been nice if they would have fixed the key/mouse rebinding bug as well.