This is a well-known problem. Two threads have discussed it. One was closed. One is still open: A compendium of OGS's terrible scoring system confusing beginners. The problem is that the AI interpolates its own judgment rather than scoring the board as it stands. The reason the whole corner is unscored is that the AI thinks the border is not technically closed. You are right that the game should be annulled (game results cannot be reversed). You can file a report from the game tab (click the triangle on the right-hand panel), and a mod will annul it.
Edit: I am now late, I see. I write so slowly.