iOS 17: iMessage attachments don't restore properly or at all from any restore method

In migrating my old phone to a new one, iMessage attachments are not being carried over fully or in most cases, at all. Neither I nor Apple Support can reach any kind of resolution. After a few hours they gave up completely, so I thought I'd turn to the community before presuming the data lost to database/index corruption (the most likely cause given the result of my last-ditch attempt).

I have an iPhone 13 and an iPhone 15 Pro. I do not use iCloud for backups or for iMessage. For migration, I followed this procedure:

1. Backup old iPhone to computer
2. Turn on new iPhone
3. Use Quick Start migration
4. This migration did not include any of my iMessage attachments, so I reset the phone
5. Restored from computer backup
6. iMessage attachments still missing -- but in iPhone Storage, I could see the space they were taking up under Messages
7. Spoke with Apple, and conducted a series of factory resets, quick start migrations, and backup restorations with no effect
8. Purchase large iCloud plan
9. Backup old phone to iCloud
10. Restore new phone from iCloud
11. Some recent attachments appear in the Media area, but most don't
12. Scrolling through messages, the photos do now all appear to be in the thread, but not picked up in the Media area. They have a download button to their right that causes the photo to be downloaded to Photos.

So in the end, the iCloud plan came the closest to some sort of restoration of the attachments. Some of the questions that came to my mind during the process were:

1. Does the computer backup include iMessage attachments?
2. Does the quick start migration include iMessage attachments?
3. Why did the new phone show the space consumed by the attachments when they weren't accessible?
4. Why did the iCloud restore get partial results the others didn't?
5. Why are the images still in the threads post-iCloud restore?

My personal takeaway here is that Apple expects everyone to use iMessages in iCloud, and any other means of keeping your messages and their attachments is not supported at all. Some sort of database/index corruption seems nearly certain here, but I don't expect there to be a user-facing way to resolve that.

If anyone has any other ideas to try, I'd be open to them.

Apple Case Number: 102099670509