A traditional notion that is obstinately held although it is unreasonable WordNet 3.0 & Open English WordNet

A mumpsimus is a "traditional custom obstinately adhered to however unreasonable it may be", or "someone who obstinately clings to an error, bad habit or prejudice, even after the foible has been exposed and the person humiliated; also, any error, bad habit, or prejudice clung to in this fashion". Wikipedia

obstinate adherence to custom, habit or error Wikidata

A person who obstinately adheres to old ways in spite of clear evidence that they are wrong; an ignorant and bigoted opponent of reform. Wiktionary