A razão é dada em o patch removendo esta opção :
laptop-mode-tools remounts file systems from within a udev rule to apply certain mount options. With MountFlags=slave, those mounts then become private to the systemd-udevd namespace and are no longer accessible from outside the namespace. While the root cause is the broken behaviour of laptop-mode-tools, with mount propagation turned off, this can result in a read-only root file system. Therefore revert the relevant parts from commit c2c13f2df42e0691aecabe3979ea81cd7faa35c7 to re-enable mount propagation for udevd.