I’m assuming you’re using an at home scale that gives you a fat percentage? They’re, at best, a guess and anything will change the results on them
I wouldn’t worry what percentages it’s giving you and just focus on lifting and getting your protein
To my understanding the only way to maintain muscle is to keep a high protein intake. Which is difficult to do on one meal a day. So yes you will most likely lose muscle. How much depends on how much protein you took prior. If you’re a regular person just trying to keep on muscle, If works for you. If you’re a bodybuilder who need huge amounts of protein, then it’s not.
enough protein. and maybe the 20 hour fast and 4 hour feeding window? break your fast with a protein-shake and later your big meal with also a lot of protein. That way you can divide your protein intake.