Fortress Climb Deluxe is a game that has absolutely nothing to do with undertale and deltarune, it is a completely separate project, then why is this game in a series of fan-made undertale?

The game was created as an expanded version of a minigame in Undertale Green, making it undertale related

if I do something like this in the form of a game and add a heart, then it will be a fan game by deltarune, since it is an expanded version of the minigame

Explain to me how adding a heart to a generic crossword puzzle would be considered a fangame?

This is an extended deltarune puzzle.

If you create an extended version of a minigame from a fan-made undertale game, you are creating a fan-made game from a fan-made game, not a fan-made undertale game