I can feel the hot, hot wafts of NW England off this.

The DIWhy. https://infosec.pub/post/1505018

I don’t think this is a mistake.

When the setting was first released it was styled Plane Scape and though has subsequently been used as Planescape the registration may date as far back as the initial early 90s.

Trademarks are a funny thing though, take Coca-Cola. If they register only COCA-COLA with the hyphen, that doesn’t allow Pepsi to make a sparkling brown sugar drink called Coca Cola. Intention is important in these matters, not just the technicality of what is registered. Registrations also allow for “stylisation”, which means you don’t need to register a new mark to stylise your existing mark.