After the release of 1.12, the composter stopped accepting baked potatoes as an item to be composted.
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Composters won't accept bamboo, either.
This happened to me also. Turns out that furnaced cooked items wont be accepted by the composter. So this bug also includes the dried kelp. But it will accept it if it was taken from the creative inventory, but not the the manually cooked ones.
P.s
Cam u add this to the report? I was going to make one but i saw yours.