Se Ultimate Discovery

So I went through the trouble to get all of the SE discoveries to find the ultimate one which is locked behind EO. Which initially I thought, whatever, I've already beaten it. But when I tried to get through the back door it says "I shoudln't explore thid area cuz somethin somethin scary somethin somethin" seemingly implying I haven't beaten the dungeon, which I have.

So is this a bug and I'm stuck here untill it's fixed, or is this really how this works? If the latter, why the fuck does it work this way?