One of the all time worst shots

I think maybe your memory is playing tricks. There are no medium shots of Mother Gin Sling without background detail, There are some relative close ups (just wide enough to get in the whole of her elaborate head dress) without set details in the b.g. but in these there is always background shadowing. I imagine that background set details were kept out of frame so as to not make the shot look too busy and to avoid conflicting with the outline of the headdress.
I know what you mean about the relative simplicity of the background in these shots (the close ups of Poppy are similarly treated but it's not so obvious because the framing is closer) but they are never as unaesthetic as you suggest.