Suggest updating to current Plex Media Server release and using the new Plex Movie agent.
Plex implemented a new agent starting with the 1.20 releases.
You must be using the new agent for Plex to pick up the IMDB/TMDB IDs in the folder and file names.
With the new agent, Plex prioritizes the IMDB/TMDB IDs. You could name the file Gone With The Wind (1939) {imdb-tt0095441}.mkv and Plex will still match it to The Cold Summer of 1953.
After updating to the current PMS release, follow Upgrading a Movie library to the use the new Plex Movie Agent to use the new agent with existing libraries. Advanced settings Plex Movie Agent has additional details regarding the new agent. Note that the advanced settings are located in the Advanced Settings of each movie library, not in the traditional Server Settings → Agents location.
After updating to PMS 1.20+, the old agent is still available. It is now known as Plex Movie (legacy), and settings are still in the Server Settings → Agents location.
Edit: Adding screenshots showing PMS prioritizing IMDB ID over folder/file name.
Also, on my system, the new movie agent correctly matched The Cold Summer of 1953 (1988).mkv, without using the IMDB ID.