Open the Plex web client and navigate to Settings -> Agents -> Movies -> Plex Movie and demote “Local Media Assets” in priority (if you haven’t already). Something like this:
You may need to do this for “The Movie Database” as well, if you use that as your agent. This will cause Plex to prefer the online sources of metadata rather than the embedded tags (which I understand you have deleted). After doing this, you may need to perform the Plex Dance to ensure any cached information related to the original match is removed:
