Description of problem:
PXE booting on PPC fails, drops to grub prompt

Version-Release number of selected component (if applicable):
Starting from grub2-2.06-54.el9 until the present (2.06-61.el9)

How reproducible:
always

Steps to Reproduce:
1. Set up PXE server on ppc
2. Try to boot client
already described in these bugs:
https://bugzilla.redhat.com/show_bug.cgi?id=2173015 (CentOS)
https://bugzilla.redhat.com/show_bug.cgi?id=2209435 (fedora)

Actual results:

1. virsh start client --console
   Domain 'client' started
   Connected to domain 'client'
   Escape character is ^] (Ctrl + ])
   Populating /vdevice methods
   Populating /vdevice/vty@30000000
   Populating /vdevice/nvram@71000000
   Populating /pci@800000020000000
   00 0800 (D) : 1af4 1000 virtio [ net ]
   00 1000 (D) : 1b36 000d serial bus [ usb-xhci ]
   00 1800 (D) : 1af4 1003 virtio [ serial ]
   00 2000 (D) : 1af4 1002 legacy-device*
   00 2800 (D) : 1af4 1005 legacy-device*
   No NVRAM common partition, re-initializing...
   Scanning USB
   XHCI: Initializing
   Using default console: /vdevice/vty@30000000

Welcome to Open Firmware

Copyright (c) 2004, 2017 IBM Corporation All rights reserved.
This program and the accompanying materials are made available
under the terms of the BSD License available at
http://www.opensource.org/licenses/bsd-license.php

Trying to load: from: /pci@800000020000000/ethernet@1 ...
Initializing NIC
Reading MAC address from device: 52:54:00:de:8d:ab
Requesting information via DHCP: done
Using IPv4 address: 192.168.124.101
Requesting file "/boot/grub2/powerpc-ieee1275/core.elf" via TFTP from 192.168.124.2
Receiving data: 321 KBytes
TFTP: Received /boot/grub2/powerpc-ieee1275/core.elf (321 KBytes)
Successfully loaded
Welcome to GRUB!

error: ../../grub-core/script/function.c:119:can't find command `source'.

GRUB version 2.06

Minimal BASH-like line editing is supported. For the first word, TAB
lists possible command completions. Anywhere else TAB lists possible
device or file completions.

grub>

Expected results:
system boots

Additional info:
If default grub.cfg file in /var/lib/tftpboot/boot/grub2/powerpc-ieee1275/ (which is a one-liner: source boot/grub2/grub.cfg) is replaced by the actual grub.cfg file, then grub menu is loaded and failure looks like this:

Booting `install RHEL-9.2'

error: ../../grub-core/script/function.c:119:can't find command `linux'.
error: ../../grub-core/script/function.c:119:can't find command `initrd'.

Press any key to continue...