This Widespread iOS 15 Messages Bug Permanently Deletes Your Saved iPhone Photos
Attention fellow iPhone users, if you have any precious photos nestled inside text messaging threads, be careful not to delete those threads, even if you have saved those photos to your camera roll. Otherwise, you risk losing those pictures forever. The culprit is a weird bug in iOS 15 and/or iCloud, and as of the latest incremental beta update, it appears to still be present.
Normally, if you save a photo from an text messaging conversation to your camera roll, it won't go anywhere until when and if you decided to manually delete it. And that is how it should be, as that is the whole point of saving a photo (well, that and the convenience of not having to hunt around message threads for a particular picture).
However, iPhone users have reported losing their photos anyway, after iCloud performs a backup. This can happen whether you perform a manual iCloud backup, or have it configured to back up your device automatically, as many people do.
The folks at MacRumors say they were able to reproduce the bug on an iPhone running iOS 15.1 beta 2. They saved a photo from a messages conversation to their camera roll, then deleted the text messaging thread and performed an iCloud backup. After the backup was finished, the the photo disappeared from their camera roll.
For whatever reason, the photo stays linked to the messaging thread even after saving it. That's a potential problem for iPhone users who routinely delete conversations and back up their contents to iCloud. If that describes you, for the time being refrain from deleting threads containing photos that you don't want to lose. I suspect Apple will get around to squashing this bug, but right now it hasn't.
Alternatively, check out our guide on how to transfer iCloud photos and videos to Google Photos. After all, you can never have too many backups. Just be aware that if iCloud backup runs before you've transferred any photos elsewhere for safe keeping, you can still lose them.