I've recently encountered the same issue. As in, the exact same. My trainer battle BGM doesn't work for any trainers, save for a BUG CATCHER. I attempted reimporting the music files, testing to see if other extensions would work, changing the unique BGM for each trainer, each map and even trying to play a BGM with a script. Nothing worked.
I even copied my entire game's data to the updated version of Essentials, but I am having the same issue. Can anyone help us?
I'm pretty sure it isn't down to the bitrate of the BGM files, seeing as my desired BGM works in a battle with a bug catcher, but not for a youngster, firebreather, cooltrainer, etc.