When you add the Artist name, the lock icon on the field turns orange. Make sure to leave it this way. Orange means “locked.” When the lock is enabled, refreshes should not overwrite your changes. (There have been some incidents where the lock fails to protect the field. I’m not sure what the status of that issue is.)
If you are confident in the completeness and accuracy of your embedded tags, you might consider configuring Plex to “prefer local metadata,” if you haven’t already.
