you should be able to use https://lua-api.factorio.com/latest/types/ItemProductPrototype.html#ignored_by_productivity in the recipe prototype to have it ignore productivity even if there is productivity on the furnaces. you would set it to the amount the recipe makes ie:

results = {
            {type = "item", name = "fish-dehydrated", amount = 1, ignored_by_productivity = 1}
        }

Ok so I did even more testing and was more diligent in testing and compared outputs of an electric furnace with prod and one without prod and using ignored_by_productivity on a recipe and ignored_by_productivity does work!

I wish that wube would change it so that the productivity progress bar would go away on recipes that have ignored_by_productivity. This gives a false impression that prod is working but in reality it is not when ignored_by_productivity is enabled. But hey at least electric furnaces work with dehydrating and thawing now.

I will add this in 1.0.5