The achievement should be fixed. (Sorry about that, it was a library loading issue unique to Mac.)
If you've already beaten the game on Mac, going to a completed file and walking doing the very last "maze" will unlock the achievement.
Quick fix for Mac Achievement
Build and release context
Steam update times within 24 hours of this post. Matching dates do not confirm which build it describes.
Build 22382749 · public branch
Steam updated · First observed