I've bought a game on Steam and completed 100% of the achievements for it a few months ago. However, I recently noticed that the number of achievements and perfect games hasn't updated on the 'Achievement Showcase' of my profile.
My best guess would be that this is due to the fact that the game in question (Lovers' Smiles) has a huge number of achievements compared to the amount of time it took me to complete them (100 achievements for barely 2 hours of playtime).
Why are some games excluded from the achievement showcase? Why do some games have achievements at all if they are not going to count towards the Achievement Showcase? Is there any way to tell before purchase if a game is in that situation?