Why do some Steam game achievements not show on the Achievement Showcase?

Back on May 2017, Steam restricted the ability of newly released games to drop Trading Cards. Then on June 2018, Valve decided to restrict more features from newly released games, this time the restrictions being:

• Limit the game's total number of achievements to 100
• The number of achievements won't count for the total number of achievements
• Achievements can't be shown on the Profile Showcase
• The game will not count for the total number of games and will not be shown in the Profile Showcase
• Will be ineligible for coupons

As per Valve's announcement, the games will be allowed to use this set of restricted features once the "confidence metric" goes up enough via player count and interaction. For the case of Lovers ' Smile, you can see that the game has a low number of reviews and a non existent player count, probably causing the game to never reach the "confidence metric".

As pointed out by Sinatr in the comments, you can see if the game has this set of features restricted by checking the features available in the game:

If the game has the restricted features enabled, the "Profile Features Limited" message will not appear: