I think the flies are attracted to the protein in the poop, and since the feed is fermented, the birds are using more and expelling less of the protein in the feed.The molasses might be a little too rich in that mix, with the addition of the sweet feed. That could be making it produce a little more alcohol smell.
I also use deep litter in the coop and that might be a factor, as well as my birds free range for most of their food and I've never seen a fly on bird poop, so my chicken's poop might be a lot like a wild bird's. You can actually pick up one of their dried turds and it will crumble in the hand, revealing bug shells and such. It smells like soil and is richly black in color, other than the urates.
Could be all these factors that keep my chicken's poop from attracting flies, but I know that their poop did attract flies when I used to just feed dry layer mash, so I think the fermentation is the kicker.