I can’t replicate it either!
I am not using the --privileged flag with docker, so the container is not privileged.
The folders are on a Sandisk flash drive. The mount is provided by WSL 2, which sets the ownership as my WSL ubuntu user.
I did notice something odd. When I try and play the file that is actually in /music2 (but is incorrectly listed under /music), Roon attempts (and fails) to play it from the top level (/Chris Isaak); when I play the file that really is in /music, Roon correctly plays it from /music/Chris Isaak.