Yeah, replacing GRUB2 binary can help here. That was also my first approach but I wasn’t lucky with latest Ubuntu one instead of old CentOS7 GRUB2.
Keep in mind that whenever you call foreman-installer the /var/lib/tftpboot/grub2/grubx64.efi will be replaced again by the one of the system (in your case Rocky 8.7).
This is not really a FDI issue rather than a GRUB2 issue.
The problem with FDI over network is, that by default initrd contains rootfs and becomes bigger and bigger over time/version. And from several bugreports I read that GRUB2 has problems when allocating memory for huge files.
You could now start to slim down the rootfs of the FDI in order to shrink initrd size. But this approach would limit the service because of missing drivers etc.
Therefor I came up with this workaround where initrd only is loaded by GRUB2 and the rootfs is then loaded separately by the Linux system.