New iPad Air - can't save a Mobi book to Files

I frequently get books from places like Prolific Works and have the books emailed to me in Mobi format so I can read them with the Kindle app. Rather than load up the Kindle app with tons of books, I've been saving them to iCloud > Kindle in Files. This has worked fine on my older iPad but not on the new one. I get a message "This operation couldn't be completed. Operation not permitted. Operation not permitted. OK" (yes, "Operation not permitted" was shown twice!)

I just tried the same thing on my old iPad and the book was saved without a hitch.

Using Spark as my email client, I tap on the icon in the email, which opens a window that states that Spark can't open the file and an arow points to the Share icon in the lower right. I tap that and when the Share options display, I tap "save to files" which then gives me the iCloud or iPad options. I select iCloud, then select the Kindle Mobi folder, and the book is saved there.

So what's going on? I suspect that there's setting somewhere that got flipped when I transferred everything to the Air but for the life of me, I can't figure out where.

Anyone have any suggestions?

Thank you