It looks like this is not a word-by-word translation. “Si suppone” is “supposedly”, “it is supposed”, “it is assumed”. I guess in the construction with “che lei” it has to be “si suppone”, otherwise it would be “lei dovrebbe”. (?)
Right, I think this is down to how the verb suppose works.
In Italian, you can suppose a thesis or theory, or an explanation, for sure not a person. So we resort to the “si suppone…” impersonal construct to introduce the uncertain statement.
I got used to the English construct, but surely it felt odd at the beginning!
Please, don’t say “è supposta che torni per pranzo”, because, aside from the reason above, “supposta” means suppository, so it could be an embarrassing thing to say…