My 26 yr old Arab is a picky fuss-budget
He always has been but the farther into his 20's he gets, the fussier
He gets a physical twice-yearly. The vet keeps declaring that he will still be chuggin' along when my Walking Horses aren't; two of them have metaoblic issues.
Streeter is really nit-pickey about hay but the hay we have now will cause him to "wander" away from his feed pan, like the OP's horse does.
He's on Triple Crown's 30% RB so, as the OP stated, there's a certain minimum amount that has to be feed for him to get his vitamins/minerals.
He has four molars missing, so he also gets well mushed up timothy/alfalfa cubes. He LOVES those --- until he sees anybody else in the barn getting that Prime Rib hay that he finds so tasty. He forgets what he's eating and will cold stare me down until I give him hay
My theory is this is just who my Arab is but it gets worse as he gets older.
The same could be possible for the OP's horse but I would still have the vet give the horse a good physical, including some blood work just to be sure.
It's also possible there might be mild ulcer issues stopping the OP's horse from finishing its supplements. My 25 yo TWH, with equine metabolic syndrome, has hind gut ulcers plus the vet is suspicious of lipomas.
Sometimes he finishes his food at night, sometimes he doesn't. He is on daily ulcer meds for the rest of his life, but if the heat/humidity are up it seems to affect his eating anyway.
I have learned to discern when he's not feeling Up To Snuff. At night I often sacrifice feeding his Triple Crown Senior and will give him 3/4 cup of equine rice bran with a handful of crushed apples and carrots, to get his metabolic and arthritis supplements in him.
The rice bran at least keeps his energy up and lets him go right to his hay when he's done eating. While they need their supplements, they need to be munching hay more than anything