Comment 9 for bug 1691158

Now imagine what happens when sda5 is mounted "behind my back" on /boot and a new kernel is installed by the Software Updater: the new kernel ends up on sda5 and the /vmlinuz and /initrd.img "system" symlinks point the new files.
On a reboot Grub does not mount sda5: the "system" symlinks point to non-existing files and Grub fails ...

The /var/log/messages file as provided shows that systemd mounts sda5 on /boot and I really cannot think of a reason for that.