Some Japanese Twitter users have noticed and are complaining (tongue in cheek faux angry Greta style) that Apple Maps no longer displays the Sea of Japan name in English or Japanese language settings. It may be connected with recent South Korean moves to get international recognition of ‘East Sea’ instead of, or in conjunction with Sea of Japan. For reference Yahoo Japan Maps does not list any sea names, while Google Maps sticks with Sea of Japan but Google uses different place naming depending on the device region setting.
Apple might be preparing to follow the Google Map naming method with updated map assets, it might not, we shall see. Whatever the reason, removing a place name altogether is one way to avoid having to deal with a problem of what to call said place.