It calculates mana like this:
mana += 30 * food.getSaturationModifier();

instead of plugging in the saturation formula, making gains from saturation miniscule, since modifier is 0.6 for most items.

I probably should've provided more info on the issue.

Saturation formula in minecraft is: nutrition * saturationModifier * 2

Neoforge's .saturation() already provides value by that formula (so 1.21 changes were not needed), but in 1.20 saturation is still not what it should be. I made a pull request to illustrate, if that helps? #1506