[iOS Files.app] Some files do not transfer when iPhone is Backedup/Restored to/from Mac

I recently moved all my data (or so I thought) from an older iPhone to a new iPhone 15 using the Backup/Restore on a Mac. I'm missing some files that are saved in the Files.app's "On My iPhone" folder. Some of them are old from 3 years ago, some of them from today. They are all downloaded from the iOS Gmail.app. They were all mail attachments. They are PDFs, Docs, JPGs... filetype does not seem to matter.

Shouldn't all files saved File.app's "On My iPhone" root folder be saved in the encrypted backups to a Mac? And why only files downloaded from Gmail? Are these attachment files always linked back to the Gmail server? That would be ridiculous because they would disappear as soon as you delete the original Gmail message.