Hello
I have a new item to report, to help understand the problem.
Surprisingly, following a bad manipulation, the second user account, created to use Band in a box, relaunched as if it was installed for the first time (a longer launch, like a reinitialization). So, when logging in to this user account, MacOs asked me to redefine the settings as if I were opening this session for the first time.
And this is where it gets interesting because I launched and could use band in a box, without needing to have previously opened Garage Band or Logic Pro (with an Apple DLS instrument track).
Unfortunately, I couldn't reproduce this with my main session. I forced the shutdown of my main session (long press on the on/off button). Then I restarted. But Apple DLS music was still missing when Band in A Box opened.