OK, solved. I’ve had disabled the NAS package group in reForris prior to the installation (as I didn’t think I needed any of its functionalities). Apparently, this uninstalls many dependencies, including block-mount.
With the package uninstalled, LuCi System menu does not offer Mount Points, and the mSATA disk does not get mounted on boot, apparently.
After reinstalling the NAS package group and rebooting, the drive and LXC containers are back.