So this sometimes happens when the files have already been written to the /boot folder while the boot partition isn't actually mounted. You can see if this is the case by running ls -ahl /boot and seeing if there are any files in there just after a fresh boot. If there are and you haven't manually mounted the boot partition and aren't in the middle of updating then you're being affected by this bug.
To see if your boot partition is mounted run mount | grep boot, if there are no results then it isn't mounted. You can mount the partition with sudo clr-boot-manager mount-boot and take a look inside. I bet there's only the 6.3.12 and 6.1.5 kernels in there for whatever reason.
If this is the case then this issue is perfectly fixable. You'll need to empty out the /boot folder while the boot partition IS NOT mounted and then clr-boot-manager will start working again as expected. Follow these steps VERY carefully and do not run them if you are not absolutely certain that the issue is what I described above.
# Make sure the boot partition is not mounted
sudo umount /boot
# Delete everything in the boot folder
sudo rm -rfv /boot/*
# Tell clr-boot-manager to regenerate the boot entries
sudo clr-boot-manager update
# See if it worked
sudo clr-boot-manager list-kernels
