Feel your eggs getting all scrambled because you filled out your entire encyclopedia and didn't get the achievement pop-up? Here's the solution:

• Save and exit the game. Completely close out of it.
  
• Relaunch the game.
  
• Select your save file with the full card completion. It should have a little gold star on to the corner. A blue star denotes having found all eggs.
  
• Once you have control of Fynn, the achievement toast should pop up.

Please note you have to completely close the game, simply quitting to the main menu and reloading your save will not work.

An example of a save file at 100%.

Hope this helps someone!