It seems that i have a similar issue with jellyfin. I've added the user "jellyfin" to the lxc-shares group (10000), the webpage sees the mounted folder, while being root in the console i have full read/write access, but the library scan doesn't work.
Did you find a solution? Maybe i'll add also lxc root to the shared group, but at this point i guess it's almost the same issue of having a privileged container...