On Android :

With update 1.0.8, unlocked achievements remain unlocked and are no longer reset each time you open the game. Unfortunately, this patch is not retroactive: you'll have to unlock previously unlocked achievements all over again.

Our apologies!

On iOS :

Achievements are unlockable, but the pop-up that notifies you that you have just unlocked an achievement is not displayed. To check if an achievement is unlocked, you need to restart the game and check from the title screen.