This update fixes the wrongly implemented achievements. Please, validate your game files, if the game doesn't update itself.
For the people that have already done the things required for an achievement, but said achievement was not unlocked:
Go to Snowbelle Town and interact with the frozen statue next to the healing crystal. All falsely locked achievements will be now unlocked. The achievement "The Evil Banishers" cannot be unlocked this way. It will require you to defeat the last boss, again.
Apologies for this issue.
Merry Christmas and happy holidays!