Thanks for the report. We checked this against the original game: in v83 the world map for that area only links Nautilus, Sleepywood and Kerning Square. The Mushroom Castle link was added in a much later version, so it's missing here on purpose. Mark may still add a shortcut later, but that would be his call.

Update: Mark decided to add it. On the Victoria Island world map, hovering the Mushroom Castle near Henesys now lights it up, and clicking it opens the Mushroom Castle map, the same way Sleepywood works. It uses the castle link from a later official version of the world map, lined up on our map. To re-test: open the World Map on Victoria Island and click the castle next to Henesys.