Here's what I am doing -
The file in my bgm folder is DecisiveBattle1.
This is how I have evented the cutscene.
Pre Battle Loop (below)
LoadBGM DecisiveBattle1 loopstart=129764 looplength=141213
<Boss Appears>
PlayBGM DecisiveBattle1
<Boss Lines Finish>
<Battle start>
This is the new loop and also the same loopstart/looplength values in the ogg file metadata (below)
PlayBGM DecisiveBattle1 loopstart=529959 looplength=4514464
Currently this order of commands is just playing the BGM with the loopstart/looplength value that is in the ogg file metadata.
I suspect the loopstart/looplength parameters are not overriding the loopstart/looplength values in the metadata and that's why this is happening.