Where...oh, where...did you see a recipe with brown sugar in it?? Please tell me!! I've never heard of such a thing!
I feel like when you are little and having a game at school where they have you sit in a circle and whisper something in someone's ear and they have to pass it to the next kid, and then they pass it along in a whisper, and so on until it comes back to the first kid and you find out that what they originally whispered doesn't even come close to what returned to them after it passed through the circle of kids.
Not that I think I originated the FF thing, but I've never even heard of using sugar to ferment grains unless you are making bread!
Sorry to tell you this....
when the FF smells like rancid vomit is when it is most filled with the beneficial microbes that is one of the goals of feeding the FF. When next it smells like that, feed it out and just keep a rolling mix with the same stuff inoculating the next batch. After the first day of mixing fresh feed in mine, it only smells slightly of vomit...all the days after that it smells fully of vomit...because I'm using the original SCOBY that I started last year. If I take the buckets apart and take a good whiff of the fluid there it most definitely smells like vomit.
You've been throwing out the best stuff and feeding your chickens only slightly fermented feeds..which is fine, but it won't hurt to let it ferment further and feed that as well.