Here's something you might find amusing. When you have multiple versions of
Linux installed, only one of them will control the Grub. It will probably be the
last one you installed.

If you get a kernel update in a Linux which does not control the Grub, the new
kernel will not be used until you boot into the "master" Linux and run
update-grub.

My laptop is set to triple-boot, so this comes up fairly often.