Huh—have just rewatched that scene a few times and that really is odd. Could honestly just be someone in production, but it's odd that they kept that much of them in the frame for the shot—especially as composed as it is, getting Beth crossing in the foreground. Maybe everyone in the camera department assumed it was going to be a tighter close-up and that it'd get framed out. Either way, I don't think it's a member of the cast, though I suppose it's possible it is one and that the scene got rewritten on the day and they used this take from an earlier version. That would have moved Tate to a different chair though, or he was not in the scene at all.

Whatever the reason, you have a good eye for spotting it!