I’m not really sure what’s going wrong with it, but apparently the paper layout is sometimes automatically switching to multi page format on submission of single page render requests, and the choice is made “sticky”, too, so the next time you create a map from the same browser multi-page layout is pre-selected.
I’ve experienced that a few times myself now, even with witnesses looking over my shoulder when it happened, but I have no idea by what this may be triggered yet.
Unfortunately I don’t have the time to debug this either right now, but I hopefully will get this tracked down by the end of the week.
Meanwhile, if you are interested, you may subscribe to the related github issue report: