Comment # 4 on bug 1188110 from Dario Faggioli

Yes, I have also experienced this. I'll add that doing:

  sudo tukit --continue execute update-bootloader

after `pkcon update` / `pkcon upgrade-system` and before rebooting acts as a
workaround for this problem, at least for me and for a couple of other users
with which I was discussing this issue.

Of course, that's not acceptable as a solution and pkcon/PackageKit should
handle kernel updates automatically. But maybe it's something useful to know,
while we look for a proper fix