Description of problem:

Trying to configure a machine to boot atumatically on kernel-rt-debug, doesn't work.

Version-Release number of selected component (if applicable):
grubby-8.28-26.el7.x86_64
grub2-2.02-0.87.el7_9.8.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Configure to boot on debug kernel
echo "UPDATEDEFAULT=yes" > /etc/sysconfig/kernel
echo "DEFAULTKERNEL=kernel-debug" >> /etc/sysconfig/kernel
echo "DEFAULTDEBUG=yes" >> /etc/sysconfig/kernel

2. install kernel-rt-debug
yum install kernel-rt-debug

3. reboot and check kernel version

Actual results:
doesn't boot on debug kernel

Expected results:
boots using kernel-rt-debug build that got installed

Additional info:
If I use kernel-debug it boots correctly.

Same reproducer works well on RHEL-8.