I've just encountered this bug as well buts oddly enough its happened to the hydra in gods of the agean sea. i cleaved his boat and it gave me the stuff for killing him but in the cultist menu it said that I had not killed him and his location was currently unknown
Hey there, rarekingawesom1! I'm sorry to hear you encountered an issue with this cultist. Can you show me a screenshot of your Cultist Menu? Also, what platform are you on, please?