A solution to this issue has been found by someone called cidony in the Apple support discussion forums. Simple solution. This is for windows as I do not have a Mac but I imagine thee is a similar process to get rid of the temp backup that won't delete itself.
Click on the iCloud for Windows app (mine is in the systray) and click on iCloud settings. Make sure you are logged in. Click on Storage and go to Backups. You will see the temporary backup of your prior device, eg. iPhone 12 for me, which you can then delete. Instantly frees up space in iCloud for backups.
The temp backup on my phone was 16GB and I kept getting your iCloud storage is full. So this solution solved this problem. I am surprised the upper level Apple support techs did not know of this solution.
If cidony is a member of this forum, thank you for this solution.