I know the history of 'la tarte Tatin' and that 'tatin', has come to be applied, in cooking circles, to other 'upside down' tarts, but as 'tatin' is an (unofficial) abbreviation for 'tarte Tatin' should it not be une/la tatin ? The only dictionary I have found that references 'tatin' as a noun on its own, Robert en ligne, defines it as feminine.
https://dictionnaire.lerobert.com/definition/tatin