I hit this bug when upgrading my Libreboot X200 from Fedora 29 ("BIOS" setup) to Fedora 30.
The Fedora 30 upgrade script set GRUB_ENABLE_BLSCFG=true and generated a new /boot/grub2/grub.cfg that Libreboot's GRUB cannot use. It uses this specification: https://systemd.io/BOOT_LOADER_SPECIFICATION and this is the Fedora wiki page: https://fedoraproject.org/wiki/Changes/BootLoaderSpecByDefault
The workaround was to use the backup copy of the old, Fedora 29 grub.cfg by following these instructions: https://fedoraproject.org/wiki/Changes/BootLoaderSpecByDefault#Upgrade.2Fcompatibility_impact
As far as I can tell, this bug was only recoverable because I was upgrading from an earlier version (and thus had a backup old-style grub.cfg to use). A fresh Fedora 30 install, or of any other distro that adopts this config specification, will not be bootable.
I am not sure what the best solution here is, for example I see the option of Libreboot's GRUB being updated to support the new config style, or instead Libreboot chainloads the distro's GRUB.
Link to RedHat's bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1729793
Well libreboot uses grub and grub.cfg so all is working as intended :)
(Just like we don't use and bios bootloaders or efi apps and so on)
Nevertheless I will rewrite it as (obscure) feature request, but currently I don't see a place where this thing would fit in bootflow of libreboot, priority wise with respect to other sources.