Comment # 1 on bug 914814 from Bernhard Wiedemann

There is supposed to be a service running at boot
that purges kernels not matching your multiversion.kernels line.
But I can't find it on my 13.2 atm.

You could check
ls /boot/vmlinuz-*
if those kernels really are still there.

Maybe you still need to manually call
update-bootloader --refresh
to let grub2 know that something changed?