Whites looking sweet!
Dont sweat that ammania. Its not even a mystery, the whole genus has the specific need of a rich sub and very lean water column, esp macros
That one of yours would be a fat weed in those conditions. Stem like a sharpie and flat leaves 3/4" wide. Yours wasnt a happy specimen even before it began to stunt. The leaves arent supposed to be so narrow and curled up like that
Still, you might be able to grow it consistantly in its pre-stunt form. Let it go until the top stunts so bad it stops. Then cut it down and leave the stump 3-4". It'll make new side branches and usually those are better adapted to present conditions and do better (works for most stems not just ammania) You probably know that but some readers may not
Either way its not worth wringing your hands over