Hi, I encounter this issue with Synology "DS File" app and every picture or video I upload has a copy in this folder. This folder is currently 4GB and it has never been cleaned. I have reproduced the issue on other devices (with iOS 15).
The major issue is that the folder is included in the iCloud / iTunes backup since iOS 15. So I had to change my iCloud plan and pay for it. It's a big issue with money impact.
There's no need to keep these files and no need to backup it ! Under iOS 14, I could not see the issue because there was no impact on the backup. With iOS 15 release, this issue will have a very big impact for some users.
Thanks for understanding.