From what I can tell:
It’s marked by a user timer and service.
grub-boot-success.timer
grub-boot-success.service
which runs: /usr/sbin/grub2-set-bootflag boot_success
and is checked in /boot/grub2/grub.cfg
You can manually show it on a reboot by doing sudo grub2-set-bootflag menu_show_once
You should also be able to hold the shift key during boot or press F8 during boot to show it.
It might be nice to have something accessible via menu to toggle it for troubleshooting.
It might also be nice to be able to temporarily clear rhgb quiet from the boot parameters as well.