Both Minecraft cakes and Cooked Turkey from Better Animals Plus don't seem to function with Diet, despite both being tagged and cake having a 5.5% increase for sugar levels.

I mean minecraft:cake yes. The one you place on the ground and eat it by right-clicking the block.

For clarity the turkey is betteranimalsplus:turkey_cooked

WAIT, I just realised the problem. I have another mod that has overwritten the block itself.

supplementaries:directional_cake

My bad.

By cakes do you mean modded cakes or minecraft:cake? Because the regular cake should work natively. However, modded edible blocks will need specific integration.