So I've always backed up to OneDrive on all phones I've had no problem. The last three years I've backed up to OneDrive and to Samsung.
When I got my new phone in early March, I saw the feature to use Gallery/Samsung with OneDrive, so I enabled that. I don't remember if I had enabled just OneDrive before that on the new phone or if that was even an option. I did transfer stuff during the setup of the new phone so it may have started like that??
At this point I've googled it and tried all suggestions I could find. Verified my settings. Only thing I haven't done is remove OneDrive and reinstall which I was going to try.
For a few months before the new phone I was having another issue with OneDrive downloading everything to my phone even though I did want to.
Terry