Yeah I'd ask for him to square the corner up. Looks incomplete or like a mis-measurement as is.

I interpreted it to mean that it would look like the baseboard would wrap around to the staircase baseboard and look like a continuous piece. Currently looks like it's missing a corner.

Called a mitered return; that's the way it's supposed to be done.

It's not done wrong but since it's a cosmetic issue rather than a structural or mechanical issue, you can have it however you want.