I have had several droplets in the past and not had this problem before.
I have copied an existing ssh key to a new droplet (Fedora 31) and am able to login via ssh key without issue with root.
When I copy another key...
Let's first being with the usual stuff, I know you mentioned you've actually checked the permissions and other stuff like that but let me post how they should be just in case:
Your home directory ~, your...