It happened with six different PDFs I tested. I have further info on the behavior - Reader will open the intended document, then after about two seconds, it will open all at once all documents of the previous session. For example, if I have three PDFs open at once, then I close Reader and then open a different PDF, the intended PDF opens, then all at once the previous session's three PDFs open.
Windows 10 Pro x64
I don't mind sharing a video but I've never done video capture before. Is there some free software?