Been a while since I kept discus but I had a school of 8 for several years. Sounds like your water is fine, "hot and clean".
Tried unsuccessfully to get mine to eat pellets and flakes. Finally had to feed them frozen brine shrimp and blood worms. Once that got started the discus started laying eggs.
An owner of one of my LFS
rotates medicated food in with her discus. I don't know the brand, but I believe it may be garlic based.
My discus had a very aggressive pecking order. The dominate male grew from "3 to dinner plate size. The weakest fish always stayed dark and was somewhat stunted.
Your once healthy fish may not be sick just a little intimidated.
Don't know your set up, but discus respond well to dark water. Might not be a bad idea to decorate with driftwood or filter with peat moss.