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 --rbind and --make-rslave in one invocation of mount.

Without --make-rslave, the behavior was unwanted (and not successful):

• umount -l would affect the original old mountpoints, too,
• and umount -R would be affected by the busy (open) files under the original old mountpoints. (Very unexpected...)