I can confirm this is the behavior if using a kvm switch and it is not active for the rocky9 server upon reboot.
I have not found a solution other than flicking the kvm back to active for the rocky server which then continues to boot.
A quick google suggests disabling secure boot that then causes grub to work and boot without the keyboard. At least from a result I found this solved the problem for them. Perhaps not ideal, but if I find anything else, I’ll update here.
Unless there is an option in the BIOS to ignore the lack of keyboard or similar.