bitterness can come from lots of factors. many times it is the vegetable that is the cause. wash off a piece and taste it. is the actual cabbage bitter? probably means it was passed its prime.
if not, sometimes if you don’t cook the sweet rice flour long enough, the starchiness can leave a bitter aftertaste.
also, bad oysters can taste milky and bitter. that could be it, too. sorry your kimchi is bitter.
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