Late to the party again but in case anyone has built all the upgrades and it still won't work:
Sometimes there's something blocking connection to the cauldron or it's too far away or whatever. Saw it 2 days ago in my partner's game. Solution was to simply place the upgrades again, seems a bit buggy at times.

Do you usually make bread at cauldron ? :D No, i dont think so, you need oven

You need Stone Oven to cook bread and lox pie

Do you need a stone over to unlock the recipes?

You need to pick up flour.. once you touch the flour the recipes will unlock if your cauldron is the appropriate level.

works for me, can bake the lox pie in the oven after making unbaked version in cauldron. fish wraps and 2x bread work. you missed an upgrade

I would double check for having all cauldron upgrades. Spice rack, Butcher's Table, Pots and Pans. Then you should be able to make the plains food.

https://valheim.fandom.com/wiki/Cauldron

have a new game whit base 1 and only base in plains it works good

Bread requires higher tier cooking upgrades. The Spice Rack and the Butcher's table I think?

... stone oven?

You may need the pots and pans - unlocked by black metal. Also, you'll need the oven to bake your stuff.