...Okay.
I didn't even get a notification about this topic until you posted your solution, but you do appear to have figured it out, so... I'll just put out that bug fix, I guess. And yes, this plugin is meant to override the regular music commands when you've got a preloaded track, specifically so that you can override things like battle music. I use MV more, so I guess an MZ bug would have flown under my own radar, so sorry about that.
I wonder why I even capitalized it in the first place. My MV plugin has it as playLoadedBgm in all cases. *shrug*
EDIT: Though, looking at it, I found another bug in MZ. Might be a similar bug in MV? I'll look into it.