How could you overwrite a device backup? If I leave a few accumulate whenever I upgrade or restore my phones, I am shown a list of backups to choose to restore from, I never do because there are usually annoyances that I am trying to avoid on old installs, so like you I do a hard reset and start from scratch. I currently had zero device backups when my messages restored yesterday. I can see them from a web browser at onedrive.live.com, settings, device backups. One question though, I was wondering if that is for settings only,(no content) as I have no backups listed there and it says across the top there are currently no devices backing up settings to ondrive. LOL if I hadn't deleted the old ones I'd have a hundred of them with no way of knowing if any were usable!
I always seen only ONE backup per device in my OneDrive.
Every backup holds the device name, and I can change name so I could create different backups.
My 520 had always the same updated backup file during HR.
When I HR from scratch (I ignored backup during Win10 insider upgrade), phone made a fresh new backup with the same name, overwriting old one.
When I changed phone, I loaded old backup for moving my settings to new one. Doing so, new phone (650) got the old name phone (520).
So I corrected the phone name, and now I got in OneDrive two backups, one per phone