There's a problem with the package trigger for the grub2 plugin due to a change in packaging in RHEL8.

By default, we get 'lsinitrd' for the current initramfs image, and 'ls -lanR' for the entire '/boot' volume. This plugin also collects boot loader configuration for a number of legacy boot loaders that are no longer used in Red Hat products (milo, silo, elilo, yaboot).

If you are looking for configuration data for either the grub1 or grub2 boot loaders those are captured by their own plugins (grub and grub2 resp.):

# sosreport -v --batch --build --profile=boot

sosreport (version 3.6)
[...]

sosreport build tree is located at : /var/tmp/sosreport-localhost-2019-01-15-roadowq

# ls sosreport-localhost-2019-01-15-roadowq/boot
boom  efi  grub2  loader
# ls sosreport-localhost-2019-01-15-roadowq/boot/grub2/
grub.cfg  grubenv
# ls -l sosreport-localhost-2019-01-15-roadowq/etc/grub2.cfg 
lrwxrwxrwx. 1 root root 22 Jan 15 10:31 sosreport-localhost-2019-01-15-roadowq/etc/grub2.cfg -> ../boot/grub2/grub.cfg

Generally, if you're just interested in boot-relevant data use the 'boot' profile, rather than just the 'boot' plugin.

Looking on a RHEL8 system, it seems that there is no longer a 'grub2' package installed by default; this means that the trigger for the plugin does not fire, and that is the reason you don't find the other data in the report.

We need to add 'grub2-common' to the package list for the plugin, since this is always installed on RHEL8 machines using grub2:

  class Grub2(Plugin, RedHatPlugin, DebianPlugin, UbuntuPlugin):
      """GRUB2 bootloader
      """

      plugin_name = 'grub2'
      profiles = ('boot',)
      packages = ('grub2', 'grub2-efi', 'grub2-common')

To test, you can either make this change to the plugin file locally, or force-enable the plugin with '-e grub2'.