Remount of USB drive partition on /boot silently failing after umount

Question

I set up an openSUSE 42.1 installation where the /boot partition is located on a USB flash drive, while everything else is using LVM volumes inside LUKS containers. So far everything seems to work fine. I can umount and remove the flash drive during runtime (reading articles like this one was the main reason for my setup choice), but I'm unable to remount /boot during runtime: it just silently fails.

I've tried both label- and uuid-based fstab definitions (both work fine when rebooting):

# /etc/fstab

LABEL=safeboot                                                  /boot                ext3       acl,user_xattr,nofail 1 2
#/dev/disk/by-id/scsi-0JetFlash_Transcend_4GB_058F-part2         /boot                ext3       acl,user_xattr,nofail 1 2

When trying to mount the drive, everything seems to be fine (I have tried all kinds of mount options, including -a, --target /boot and more):

# mount -v --target /boot
mount: /dev/sde2 ist auf /boot eingehängt.

(translation from German: /dev/sde2 is mounted on /boot).

# mount -a -v
/boot                    : successfully mounted
/                        : ignoriert (trans: ignored)
/srv                     : bereits eingehängt (trans: already mounted)
/var                     : bereits eingehängt
/home                    : bereits eingehängt

Whatever I do: /boot remains empty, despite the messages saying everything is fine. The list of mounts (mount, /etc/mtab, /proc/mount) remains boot-free.

Mounting the device to any other path than /boot works fine, though. It seems like /boot is somehow locked from being remounted.

I'd appreciate any help to further narrow down this strange behavior.

Update to answer Tom´s questions:

findmnt /boot returns nothing, findmnt returns mount tree that contains everything but /boot.

# ls -la /boot
insgesamt 8
drwxr-xr-x  2 root root 4096  3. Mär 18:51 .
drwxr-xr-x 26 root root 4096  3. Mär 18:51 ..

# umount /boot
umount: /boot: nicht eingehängt (trans: not mounted)

The kernel has been the same for several reboots. Since the USB drive has been in place most of the times it contains a compatible version. I even re-installed the kernel and thus re-wrote initrd by running zypper install --force kernel-default some time (and reboots) ago while /boot was still mounted.