Although not a direct answer to this question, this problem has been solved and the answer may help other users having problems with kernel and/or bootloader upgrades.

In this specific case, the problem was caused by an additional USB card reader inserted into one of the free USB ports of the remote Pi, containing a 'perfect' clone of the master SD card from which the Pi boots.

Being a 'perfect' clone, the **UUID**'s of the partitions on the **master** SD card and the partitions on the **cloned** SD card were **identical**. This 'confused' the boot process, which, after booting from the master SD card, mounted the **boot** partition of the **cloned** SD card on **/boot**, while the **root** partition of the **master** SD card was mounted on '**/**'.

During an upgrade of raspberrypi-kernel and bootloader, the boot partition on the cloned SD card would get updated instead of the boot partition of the master SD card.

For more information see: [How does the pi select the device to boot from?][1]

In my case, I worked around the issue by **not** using the **UUID**'s as partition identifiers, but instead using the **partition names** (**/dev/mmcblk0p1** and **/dev/mmcblk0p2**) as identifiers in **/etc/fstab** and **/boot/cmdline.txt**

It would also have been also possible to simply change the UUID's of the cloned SD card. Note however, that the UUID's to be used as identifiers are the **partition UUID**'s, which can be displayed using: **lsblk -o name,partuuid**

  [1]: https://www.raspberrypi.org/forums/viewtopic.php?f=28&t=254754