RichardLynch
Note that the sbb ticket shop is a ‘one-stop-shop’ universal platform for 250 public transport operators (RhB as Bernina Express operator being one of them) and 20 regional fare networks, so the average traveller does not have to bother with getting separate tickets from separate places for each operator.
However it is virtually impossible to accurately depict all pecularities, irks and quirks of every single operator on such an universal platform.
For a start, railway seat reservations are generally unusual in Switzerland, then trains with mandatory reservations are even more unusual (SBB do not operate any), now just don’t let us get started with coaches without reservation in such trains…
It looks like the RhB ticket shop (= Bernina Express operator) would better map your travel scenario. But here again you have to be careful not to click on ‘Bernina Express’, but on ‘tickets without seat reservations’ instead, so you must be already aware that there are such unreserved coaches.