OSX SFP6, MacBookPro, High Sierra.
I created an observation of several double stars that showed noticeable changes in position angle from the positions reported in SkySafari and its DSS images.
These observations were therefore part of an observing session, but not assigned to an observing list.
I then created an new observing list called "Stars in Motion". I went back into the observations and into the session and the entry for the star observed. When I clicked on the "Observing List" button (which of course showed "None"), it brought up what should have been a list of observing lists. Instead, it only listed the first observation list in my list of observation lists (This object was not in the only list show, BTW). I then exited and went back to "Observing Lists" and moved "Stars in Motion" to the top of the list. When I went back to the observations and again tried to change the an observing list assigned to the observed star from "None", I again was presented only a list of one observing list, and it was now the "Stars in Motion" observing list. This work around allowed me to do what I intended, but of course it shouldn't work this way.
My presumption is that the functionality intended is that it should show all observing list, and you can select an observing list and it will add it to that observing list. Or maybe it should show the observing lists its already listed within at the top (highlighted to indicate this somehow) so you can just select one of those or you can add the observed object to a list that it isn't yet present in.
BTW, the iOS version of SFP 6 is broken even more. It does not show any observation lists at all, so there is no way of adding an link an observed object in an observation to an observing list, probably unless its already there. This is a real problem if I want to accumulate some observations unassigned to an observing list to an observing list it isn't present in. E.g. new interesting object, lets look at this again sometime. I can't even center it, get info, etc, so in iOS you are really stuck in this situation.