I did a bit of testing myself and I can’t find any pattern for which mythological characters are fictional vs which are ambiguous.

Ceridwen (a character from Welsh folklore) is ambiguous but her daughter Creirwy is not a recognised public figure. Lilith is fictional. Shamhat (from the epic of Gilgamesh) is fictional.

Edit: to muck around a bit further I tried some saints. Dwynwen is is a historical figure, Gwen the triple breasted is not. Agatha of Sicilly is corrected into Agatha Christie for some reason That was 'cos of a typo, it does recognise her as a historical figure.