Things appear normal initially:

gene@Mobile-PC:~> sudo mount -v UUID=4c96a0c2-f72f-4827-b023-83ba000b2fd2
/mnt/chroot
mount: /dev/sdb2 mounted on /mnt/chroot.
gene@Mobile-PC:~> sudo mount -v --rbind /sys /mnt/chroot/sys
mount: /sys bound on /mnt/chroot/sys.
gene@Mobile-PC:~> sudo mount -v --make-rslave /mnt/chroot/sys
mount: /mnt/chroot/sys propagation flags changed.
gene@Mobile-PC:~> sudo umount -v -R /mnt/chroot
umount: /mnt/chroot/sys/kernel/security unmounted
umount: /mnt/chroot/sys/fs/cgroup (cgroup2) unmounted
umount: /mnt/chroot/sys/fs/pstore unmounted
umount: /mnt/chroot/sys/firmware/efi/efivars unmounted
umount: /mnt/chroot/sys/fs/bpf (bpf) unmounted
umount: /mnt/chroot/sys/kernel/debug unmounted
umount: /mnt/chroot/sys/kernel/tracing (tracefs) unmounted
umount: /mnt/chroot/sys/fs/fuse/connections (fusectl) unmounted
umount: /mnt/chroot/sys/kernel/config unmounted
umount: /mnt/chroot/sys unmounted
umount: /mnt/chroot unmounted

gene@Mobile-PC:~> sudo mount -v UUID=4c96a0c2-f72f-4827-b023-83ba000b2fd2
/mnt/chroot
mount: /dev/sdb2 mounted on /mnt/chroot.
gene@Mobile-PC:~> sudo mount -v --rbind /sys /mnt/chroot/sys
mount: /sys bound on /mnt/chroot/sys.
gene@Mobile-PC:~> sudo mount -v --make-rslave /mnt/chroot/sys
mount: /mnt/chroot/sys propagation flags changed.
gene@Mobile-PC:~> sudo umount -v -R /mnt/chroot
umount: /mnt/chroot/sys/kernel/security unmounted
umount: /mnt/chroot/sys/fs/cgroup (cgroup2) unmounted
umount: /mnt/chroot/sys/fs/pstore unmounted
umount: /mnt/chroot/sys/firmware/efi/efivars unmounted
umount: /mnt/chroot/sys/fs/bpf (bpf) unmounted
umount: /mnt/chroot/sys/kernel/debug unmounted
umount: /mnt/chroot/sys/kernel/tracing (tracefs) unmounted
umount: /mnt/chroot/sys/fs/fuse/connections (fusectl) unmounted
umount: /mnt/chroot/sys/kernel/config unmounted
umount: /mnt/chroot/sys unmounted
umount: /mnt/chroot unmounted

However, if the external USB connected NVMe SSD is unplugged/re-plugged between
the two successive mount sequences, the second one fails.

gene@Mobile-PC:~> sudo mount -v UUID=4c96a0c2-f72f-4827-b023-83ba000b2fd2
/mnt/chroot
mount: /dev/sdb2 mounted on /mnt/chroot.
gene@Mobile-PC:~> sudo mount -v --rbind /sys /mnt/chroot/sys
mount: /sys bound on /mnt/chroot/sys.
gene@Mobile-PC:~> sudo mount -v --make-rslave /mnt/chroot/sys
mount: /mnt/chroot/sys propagation flags changed.
gene@Mobile-PC:~> sudo umount -v -R /mnt/chroot
umount: /mnt/chroot/sys/kernel/security unmounted
umount: /mnt/chroot/sys/fs/cgroup (cgroup2) unmounted
umount: /mnt/chroot/sys/fs/pstore unmounted
umount: /mnt/chroot/sys/firmware/efi/efivars unmounted
umount: /mnt/chroot/sys/fs/bpf (bpf) unmounted
umount: /mnt/chroot/sys/kernel/debug unmounted
umount: /mnt/chroot/sys/kernel/tracing (tracefs) unmounted
umount: /mnt/chroot/sys/fs/fuse/connections (fusectl) unmounted
umount: /mnt/chroot/sys/kernel/config unmounted
umount: /mnt/chroot/sys unmounted
umount: /mnt/chroot unmounted

gene@Mobile-PC:~> # unplug/replug /dev/sdb2

gene@Mobile-PC:~> sudo mount -v UUID=4c96a0c2-f72f-4827-b023-83ba000b2fd2
/mnt/chroot
mount: /mnt/chroot: mount(2) system call failed: File exists.

Deleting/re-creating the mount point leads to a successful remount.

gene@Mobile-PC:~> sudo rmdir /mnt/chroot
gene@Mobile-PC:~> sudo mkdir /mnt/chroot
gene@Mobile-PC:~> sudo mount -v UUID=4c96a0c2-f72f-4827-b023-83ba000b2fd2
/mnt/chroot
mount: /dev/sdc2 mounted on /mnt/chroot.
gene@Mobile-PC:~> sudo mount -v --rbind /sys /mnt/chroot/sys
mount: /sys bound on /mnt/chroot/sys.
gene@Mobile-PC:~> sudo mount -v --make-rslave /mnt/chroot/sys
mount: /mnt/chroot/sys propagation flags changed.
gene@Mobile-PC:~> sudo umount -v -R /mnt/chroot
umount: /mnt/chroot/sys/kernel/security unmounted
umount: /mnt/chroot/sys/fs/cgroup (cgroup2) unmounted
umount: /mnt/chroot/sys/fs/pstore unmounted
umount: /mnt/chroot/sys/firmware/efi/efivars unmounted
umount: /mnt/chroot/sys/fs/bpf (bpf) unmounted
umount: /mnt/chroot/sys/kernel/debug unmounted
umount: /mnt/chroot/sys/kernel/tracing (tracefs) unmounted
umount: /mnt/chroot/sys/fs/fuse/connections (fusectl) unmounted
umount: /mnt/chroot/sys/kernel/config unmounted
umount: /mnt/chroot/sys unmounted
umount: /mnt/chroot unmounted

Thanks for your help,
Gene