I believe it's because of the nature of FFA:
"In case of insufficient material, threefold repetition or the 50-move rule, the remaining players will share the points of the remaining kings."
Try adding an anchor piece already placed on the board before setup begins.
If that doesn't work, it could also be because of the autoclaim win feature. The BOTs see that there are only kings, so they end the game to maximize points. They don't see that it's setup.
What happens in my game? Why bot resigning?

What happens in my game? Why bot resigning?https://www.chess.com/variants/custom/game/31428498