This matching behavior has existed for nearly a year. It was recognized shortly after the transition to TheTVDB’s version 3 API was completed (correlation does not necessarily imply causation notwithstanding). At the time, it appeared to manifest primarily when adding new 2020 shows. Here are a couple of threads which discuss the behavior:
There was some minor participation from Plex employees in both threads. They specifically mention TheTVDB’s API change as contributing to the new behavior but didn’t provide any specifics. The message was the same in both cases: If you want the best chance of having automatic matches for your TV series, name them exactly as they appear at TheTVDB.
Personally, I always include the year and Match/Fix Match as necessary.
