[Autumn]

Well more so this happens every time I've logged on to a character with custom music on, which is about all of them, I hope that counts?

Either way I tested it.

Enter arena battle with custom song and marching band, the music plays.

Win fight, song ends, no victory music as expected.

Enter another arena battle with custom song and marching band, the music no longer plays.
Turned music from 'custom' to 'on' and music started playing again, then turned it back to 'custom' and music fades out.

Entered ANOTHER arena battle after this with custom still set, music turns back on.

Entered one more arena battle after this, with custom still on, music no longer plays.

Entered one more, music isn't playing again.

I believe it plays every 'other' battle, after the victory screen.