Trying on my system manually to update kernel args for a DEFAULT kernel leads to a different behaviour based whether the system is booted in DEFAULT kernel or not. In the first case it does nothing.

Note that `grubby --info DEFAULT` is also broken this way - as visible below.

• Initial system state (booted in default kernel)

  [root@rhelmin9 ~]# uname -r
  5.14.0-503.40.1.el9_5.x86_64
  
  [root@rhelmin9 ~]# cat /proc/cmdline
  BOOT_IMAGE=(hd0,msdos3)/vmlinuz-5.14.0-503.40.1.el9_5.x86_64 root=UUID=fcb267f7-33de-4e82-a2a0-260d943a8c9f ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/swap
  
  [root@rhelmin9 ~]# grubby --default-kernel
  /boot/vmlinuz-5.14.0-503.40.1.el9_5.x86_64
  
  [root@rhelmin9 ~]# grubby --info DEFAULT ## BROKEN
  
  [root@rhelmin9 ~]# grubby --info ALL
  index=0
  kernel="/boot/vmlinuz-5.14.0-503.40.1.el9_5.x86_64"
  args="ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/swap"
  root="UUID=fcb267f7-33de-4e82-a2a0-260d943a8c9f"
  initrd="/boot/initramfs-5.14.0-503.40.1.el9_5.x86_64.img"
  title="Red Hat Enterprise Linux (5.14.0-503.40.1.el9_5.x86_64) 9.5 (Plow)"
  id="e1929802e2944ac4bc52699de6577280-5.14.0-503.40.1.el9_5.x86_64"
  index=1
  kernel="/boot/vmlinuz-5.14.0-503.11.1.el9_5.x86_64"
  args="ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/swap"
  root="UUID=fcb267f7-33de-4e82-a2a0-260d943a8c9f"
  initrd="/boot/initramfs-5.14.0-503.11.1.el9_5.x86_64.img"
  title="Red Hat Enterprise Linux (5.14.0-503.11.1.el9_5.x86_64) 9.5 (Plow)"
  id="e1929802e2944ac4bc52699de6577280-5.14.0-503.11.1.el9_5.x86_64"
  index=2
  kernel="/boot/vmlinuz-0-rescue-e1929802e2944ac4bc52699de6577280"
  args="ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/swap"
  root="UUID=fcb267f7-33de-4e82-a2a0-260d943a8c9f"
  initrd="/boot/initramfs-0-rescue-e1929802e2944ac4bc52699de6577280.img"
  title="Red Hat Enterprise Linux (0-rescue-e1929802e2944ac4bc52699de6577280) 9.0 (Plow)"
  id="e1929802e2944ac4bc52699de6577280-0-rescue"
  

• booted in default kernel, trying to update default kernel (nothing happens):

  [root@rhelmin9 ~]# grubby --update-kernel=DEFAULT --args "new_arg_default_kernel=DEFAULT"
  
  [root@rhelmin9 ~]# grubby --info ALL
  index=0
  kernel="/boot/vmlinuz-5.14.0-503.40.1.el9_5.x86_64"
  args="ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/swap"
  root="UUID=fcb267f7-33de-4e82-a2a0-260d943a8c9f"
  initrd="/boot/initramfs-5.14.0-503.40.1.el9_5.x86_64.img"
  title="Red Hat Enterprise Linux (5.14.0-503.40.1.el9_5.x86_64) 9.5 (Plow)"
  id="e1929802e2944ac4bc52699de6577280-5.14.0-503.40.1.el9_5.x86_64"
  index=1
  kernel="/boot/vmlinuz-5.14.0-503.11.1.el9_5.x86_64"
  args="ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/swap"
  root="UUID=fcb267f7-33de-4e82-a2a0-260d943a8c9f"
  initrd="/boot/initramfs-5.14.0-503.11.1.el9_5.x86_64.img"
  title="Red Hat Enterprise Linux (5.14.0-503.11.1.el9_5.x86_64) 9.5 (Plow)"
  id="e1929802e2944ac4bc52699de6577280-5.14.0-503.11.1.el9_5.x86_64"
  index=2
  kernel="/boot/vmlinuz-0-rescue-e1929802e2944ac4bc52699de6577280"
  args="ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/swap"
  root="UUID=fcb267f7-33de-4e82-a2a0-260d943a8c9f"
  initrd="/boot/initramfs-0-rescue-e1929802e2944ac4bc52699de6577280.img"
  title="Red Hat Enterprise Linux (0-rescue-e1929802e2944ac4bc52699de6577280) 9.0 (Plow)"
  id="e1929802e2944ac4bc52699de6577280-0-rescue"
  

• Install new kernel, stay booted in old one, DEFAULT starts to work

  [root@rhelmin9 ~]# grubby --info DEFAULT
  index=0
  kernel="/boot/vmlinuz-5.14.0-570.12.1.el9_6.x86_64"
  args="ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/swap"
  root="UUID=fcb267f7-33de-4e82-a2a0-260d943a8c9f"
  initrd="/boot/initramfs-5.14.0-570.12.1.el9_6.x86_64.img"
  title="Red Hat Enterprise Linux (5.14.0-570.12.1.el9_6.x86_64) 9.6 (Plow)"
  id="e1929802e2944ac4bc52699de6577280-5.14.0-570.12.1.el9_6.x86_64"
  
  [root@rhelmin9 ~]# grubby --info ALL
  index=0
  kernel="/boot/vmlinuz-5.14.0-570.12.1.el9_6.x86_64"
  args="ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/swap"
  root="UUID=fcb267f7-33de-4e82-a2a0-260d943a8c9f"
  initrd="/boot/initramfs-5.14.0-570.12.1.el9_6.x86_64.img"
  title="Red Hat Enterprise Linux (5.14.0-570.12.1.el9_6.x86_64) 9.6 (Plow)"
  id="e1929802e2944ac4bc52699de6577280-5.14.0-570.12.1.el9_6.x86_64"
  index=1
  kernel="/boot/vmlinuz-5.14.0-503.40.1.el9_5.x86_64"
  args="ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/swap"
  root="UUID=fcb267f7-33de-4e82-a2a0-260d943a8c9f"
  initrd="/boot/initramfs-5.14.0-503.40.1.el9_5.x86_64.img"
  title="Red Hat Enterprise Linux (5.14.0-503.40.1.el9_5.x86_64) 9.5 (Plow)"
  id="e1929802e2944ac4bc52699de6577280-5.14.0-503.40.1.el9_5.x86_64"
  index=2
  kernel="/boot/vmlinuz-5.14.0-503.11.1.el9_5.x86_64"
  args="ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/swap"
  root="UUID=fcb267f7-33de-4e82-a2a0-260d943a8c9f"
  initrd="/boot/initramfs-5.14.0-503.11.1.el9_5.x86_64.img"
  title="Red Hat Enterprise Linux (5.14.0-503.11.1.el9_5.x86_64) 9.5 (Plow)"
  id="e1929802e2944ac4bc52699de6577280-5.14.0-503.11.1.el9_5.x86_64"
  index=3
  kernel="/boot/vmlinuz-0-rescue-e1929802e2944ac4bc52699de6577280"
  args="ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/swap"
  root="UUID=fcb267f7-33de-4e82-a2a0-260d943a8c9f"
  initrd="/boot/initramfs-0-rescue-e1929802e2944ac4bc52699de6577280.img"
  title="Red Hat Enterprise Linux (0-rescue-e1929802e2944ac4bc52699de6577280) 9.0 (Plow)"
  id="e1929802e2944ac4bc52699de6577280-0-rescue"
  
  
  [root@rhelmin9 ~]# grubby --update-kernel=DEFAULT --args "new_arg_default_kernel=DEFAULT"
  
  [root@rhelmin9 ~]# grubby --info DEFAULT
  index=0
  kernel="/boot/vmlinuz-5.14.0-570.12.1.el9_6.x86_64"
  args="ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/swap new_arg_default_kernel=DEFAULT"
  root="UUID=fcb267f7-33de-4e82-a2a0-260d943a8c9f"
  initrd="/boot/initramfs-5.14.0-570.12.1.el9_6.x86_64.img"
  title="Red Hat Enterprise Linux (5.14.0-570.12.1.el9_6.x86_64) 9.6 (Plow)"
  id="e1929802e2944ac4bc52699de6577280-5.14.0-570.12.1.el9_6.x86_64"
  

Additional info:

• grubby-8.40-64.el9.x86_64
• BIOS boot (not sure whether EFI has a different behaviour)
• BLS enabled
• Note that possibly other RHEL systems have the same behaviour, I haven't tried that anywhere else.

Note that data below are already compromised, as in the meanwhile I started to use the system already for other purposes (updated kernel, rebooted, updated kernel args, ..)

• /etc/default/grub (NOTE: I updated kernel args in the meanwhile, so ignore possible difference, like `idle=poll`:

  [root@rhelmin9 ~]# cat /etc/default/grub 
  GRUB_TIMEOUT=5
  GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
  GRUB_DEFAULT=saved
  GRUB_DISABLE_SUBMENU=true
  GRUB_TERMINAL_OUTPUT="console"
  GRUB_CMDLINE_LINUX="crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/swap idle=poll"
  GRUB_DISABLE_RECOVERY="true"
  GRUB_ENABLE_BLSCFG=true
  

• grub2-editenv:

  [root@rhelmin9 ~]# grub2-editenv - list
  saved_entry=e1929802e2944ac4bc52699de6577280-upgrade.x86_64
  boot_success=0