When using a custom playlist for BossBattle the volume for the music is MUCH lower than other scenarios.
Ambient, NightTime, Battle, etc are all at an equal level of loudness.
Mind you I have normalized my songs since I use music from different albums so there is no issue with the specific songs in the BossBattle playlist.
If I don’t use a custom playlist I get the default music, which plays at a normal volume.
I can work around it by boosting the BossBattle songs and / or lowering the gain for all the other songs, but there’s a limit to how much you can increase the gain before distortion occurs.