This worked for me correctly -- https://unix.stackexchange.com/a/264488/4319:
mount --rbind /dev /mnt/test mount --make-rslave /mnt/test umount -R /mnt/test
It was important to have the two first commands as two separate commands: do not combine
--make-rslave in one invocation of mount.
--make-rslave, the behavior was unwanted (and not successful):
umount -lwould affect the original old mountpoints, too,
umount -Rwould be affected by the busy (open) files under the original old mountpoints. (Very unexpected...)