I finished the game, unlocked all the cards, and Grandpa is gone. I can't find any info about this online other than deleting the save and beating the game again. How do you keep Grandpa from going away to do this category?

I kinda solved it. He seems to only show up on certain days or something.

He can also disappear if the save file is moved or missed with. I think it has something to do with anti piracy