I've completed everything with no hints, but it doesn't show up.

Ate to play on win 10 did not seem to find any problems. If there are problems with Normal mode on win 8.1: it is not passable, it writes about the arrival of a train with cats and problems with the menu begin (you can’t exit, restart, etc. Exit ONLY through a forced alt+f4 termination)....

I missed the no hints acheievement for the hard main level because the cat was invisible, this doesn't feel fair

has happened w other hidden cat games, but i have turned off all sounds on game, it doesnt work, with 1 game it turns on randomly even tho its still turned off

Found all cats in Advanced: https://imgur.com/a/JtZDg3X Achievement did not unlock. I did use some hints. Last cat I found was Rita Oreo, if that matters.