I highlight two documents in the binder and my editor will quite correctly show me my scrivenings of these two to allow me to edit as one.
But if I enter full screen without clicking in the edit window I lose the scrivening and am left editing one document. To retain my scrivening I have to make sure the edit window is selected before Alt-Cmd-F.
You should download the new update, but it’s not going to change this, which is a feature rather than a bug. The full screen command is designed to take a list of files in the binder and create a history queue of them, so you can flip between documents easily in full screen with Cmd-[ and ]. What happens to be showing up in the editor is completely incidental. You just happened to have Scrivenings mode turned on, but you could have just as easily have had corkboard turned on. Calling full screen from the editing session, on the other hand, will retain whatever editor setup you have going on, even if it has nothing to do with the current binder selection.