What is “set body weight”?
It works the same as everything else. If you eat more than you spend, you’ll gain weight. If you stop fasting, chances are that you will eat more than you did when on a fasting regimen. So… yeah.
Longer fasts mostly make it easier to eat a reasonable amount of calories. Yes, fasting has a number of physiological advantages that may give you more leeway than other diets, but I feel like there’s a point of diminishing returns. Doing 20:4 instead of, say, 18:6 may help but not as much as eating what your body needs and not more.