I think it has learned that stones completely within your own ring of stones are dead and can just be assumed part of your own eyespace. It has just failed to learn the exceptional case where such stones form 2 eyes.
Funny, seeing with two eyes is needed for depth perception. Pretty sure that was a really subtle joke from the user above you :)
Oh, (only) now I see the joke:-!
I was somehow relating the word depth to search depth...