The reason that Sausage counts as Butter is that it's given the name "Butter" in the code:
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It does have a unique ID, but things go wrong when it's compared by name.
You can fix it by replacing "Butter" by "Sausage", but you have to start a new game so all the object references are initialized correctly (might be possible to redefine it or update the existing objects, I didn't look too deep).
Even with that fix, I don't think the fridge shows the correct amount of ingredients. I got the message that mom couldn't cook due to lack of ingredients, when the fridge looked like it had a ton of stuff in it. Trying to cook myself then lead to an error.