This bug is about to become a much bigger problem. As of e2fsprogs 1.47.0, metadata_csum_seed is enabled by default.

You can see release notes here:
https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git/tree/doc/RelNotes/v1.47.0.txt

So default arguments mkfs.ext4 is now no longer compatible with GRUB ext4 detection.

Gentoo has backported this patch: https://git.savannah.gnu.org/cgit/grub.git/patch/?id=7fd5feff97c4b1f446f8fcf6d37aca0c64e7c763

As far as I can tell, that patch has not yet made it into a released GRUB version.

Details from report of this bug for metadata_csum_seed:

https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/1844012

/usr/lib/grub/x86_64-efi/ext2.mod, from:

grub-efi-amd64-bin:
  Installed: 2.02-2ubuntu8.13
  Candidate: 2.02-2ubuntu8.13

doesn't recognize an ext4 filesystem with metadata_csum_seed set.

This is true in command-line grub, not just the update-grub software. Attempting

linux (hdx,gptx)/ etc

on an ext4 fs with metadata_csum enabled fails; removing the metadata_csum_seed option fixes this.

The failure is "unknown filesystem" for any ext4 fs with metadata_csum_seed set. Apparently saving the metadata checksum seed in the superblock changes it beyond recognition for grub.

 The message appears to be coming from
grub-2.04/grub-core/kern /fs.c:

grub_fs_t
grub_fs_probe (grub_device_t device)
{....
 grub_error (GRUB_ERR_UNKNOWN_FS, N_("unknown filesystem"));
}
which appears to probe for filesystems by calling the fs modules to attempt to mount it.

I tried to install latest grub from eaon to see if upstream has fixed this, but it depended on too much other unreleased software such as libc6.

I pulled the 2.04 source. There is nothing in the Changelog that appears to address this.

from `man ext4`:
 metadata_csum_seed
              This feature allows the filesystem to store the metadata checksum seed in the superblock, which allows the administrator to change the UUID of a filesystem using the metadata_csum feature while it is mounted.