There are several kinds of buffs provided by food in the game. There are defensive and offensive food, which raises stats of things like defense, attack, crit rate, and crit damage. There are utility consumables which up specific elemental damage, or increases resistance to particular elements for a short duration, or decreases stamina consumption or restores flat stamina.
Do all these buffs stack? Are they all treated as separate instances of buffs? Or do some buffs override each other? For instance, do attack buffs from food stack? Does a greater amount of attack always override another attack buff? Does the same logic apply for hybrid foods which apply offensive buffs in multiple categories (ie Adeptus Temptation, which buffs both Attack and Crit Rate).
Best Answer
TL;DR:
I did the following test: I consumed food items one after another and took notes of my char stats:
In experiment #2 we see that offensive and defensive buffs can be active at the same time.
In experiment #3 we see that different offensive buffs do not stack - Critical Chance buff replaces Attack buff.
In experiments #4-7 we see that each consequent offensive buff overrides previous. And unfortunately 2* can override 4*.
Stamina reduction food has their own category, and it's possible to keep all 3 categories buffs up at the same time, see buff icons are present for current character and for the entire party:
Regeneration items can be used at the same time, but they don't have any icon - you can only see green glow around your character.