I can't give you any good reason why this happens with "intermittent fasting" in particular, but I can tell you that it follows a similar pattern to every other

I can't give you any good reason why this happens with "intermittent fasting" in particular, but I can tell you that it follows a similar pattern to every other fad diet. Almost everyone who follows them starts controlling what they eat, focussing on losing weight and they generally succeed in doing so for the first ~6 weeks. Then for whatever reason, a small number continue to lose weight and the remainder (the majority) plateau and inevitably fall off and give up. There are those who these diets work for, and some of those near religiously insist that it's simple and it should work for everyone and that if it isn't working then that's probably down to some personal failing. But the truth is that this is a pattern and it's not specific to any given diet, and it's not necessarily down to someone doing something wrong. For all that we think we know about diets, fat and weight - we haven't quite cracked weight loss and it's certainly not as simple as measuring calories in/out.