I don’t have any specific experience with this issue myself, so this is just an idea. Depending on which agent you’re using, there’s either an option to “Find trailers and extras automatically” or “Find extras.” Try disabling the appropriate option and then refresh metadata. After the refresh completes, see if they’re gone. Then re-enable the feature and refresh again.
Oh, and obviously, before doing this I would ensure I had a good database a backup (well, I’d ensure I had the entire Plex application data folder backed up).
The trailers in the new library (let’s call it library C) look exactly the same as in the “legacy library” (let’s call it libary B) which I added the movies today to.
Those English trailers entered my “library A” years ago. Refreshing metadata on them does not delete any trailers in library A.