Here is another example of "plurimos" being on either side of the noun. "Plurimos mores habet." adjudicated correct.
I also added 'is' to the beginning of the sentence to clarify that it is a he, and not a she, who 'has very many customs'.
Ei (dative of possession) sunt plurimi mores
I thought so too! Dont corget to report it so they can fix it.