Preview will not save merged PDFs

I'm combining three PDF files into one, something I have done many hundreds of times. All of a sudden (since the last time I tried a month or so ago), I merge the files, save it, close it, and then when I reopen it, I'm back to only the first document!

The first document requires an inserted blank page, and that seems to be causing the problem--and honestly, this has never happened before, and I've been doing this for years. As an experiment, I tried merging the first document AFTER I had merged the other two, and then inserting the blank page. On reopening, the blank page is gone.

What do I have to do to get this to behave normally?

Thank you,

Mark