Preview in Catalina Not Displaying Dual-Page View Correctly *AND* Unable to Save Changes

I have a bunch of Loeb Classical Library books in PDF legally downloaded from the web. Today, when I opened a few of them to see what they look like, I was surprised to find that the original Latin or Greek texts do NOT face their corresponding English translations, as they were supposed to!

IIRC, in earlier versions of MacOS, I did not encounter this problem. I then tried to insert a blank page into the PDF doc and solved the problem of the Latin or Greek texts do not face their English translations. However, Preview was not able to save the changes, even though I specifically chose to save the change--the inserted blank page, that is.

I did some search. There was a thread with 9 replies about not being able to save an inserted blank page into a PDF doc. I was going to chime in to that thread, but the reply button was mysteriously greyed out, so I am forced to start a new thread.

Any info would be appreciated!