I'm really sorry that I missed this error, but it's fixed now!
Steam achievements are already working, if you start the game in the level selection menu it will check which ones you should have and activate them.
Thank you very much for playing!
Steam Achievements fixed!
Build and release context
Steam update times within 24 hours of this post. Matching dates do not confirm which build it describes.
Build 11496570 · public branch
Steam updated · First observed