[MothEnthusiast]

When you have a marching band and set your options to 'music: custom only', you can hear your marching band music just fine in normal fights, but only silence in Korvara boss fights. Haven't tested with G6 bosses. My guess is that it's prioritizing boss music over the player's music, but since that's not custom, it plays nothing instead.

Which, not 100% certain if that would count as a bug, but it is disappointing since those tend to be the longest fights.

Tested on Winged Serpent and Bandit Boss.

[Neus]

This is not a bug, boss themes override marching bands. If you have your music set to custom only, it won't play anything because the battle theme is set to the boss's theme.