I was in the middle of installing Funtoo on a new drive, in MBR mode. Wnen I got to installing grub, the following happened:
grub-install --target=i386-pc --no-floppy /dev/nvme0n1 (on which / is, specifically /dev/nvme0n1p4, formatted ext4 using the live USB utilities just a few minutes prior)
grub-install: error: unknown filesystem

Looking for a solution, since this had never happened before in all the installs I did in the past, and found this:
https://www.linuxquestions.org/questions/slackware-14/grub-related-issue-with-alienbob%27s-current-64-bit-isos-clean-fresh-install-4175722697/#post6417052
someone having the same issue installing Slackware/grub with an ext4 partition.
The suggested solution:
tune2fs -O ^metadata_csum_seed /dev/sdX
worked for me. After running that and then attempting grub-install, it worked.

If I understand the issue correctly, as I found somewhere else, "When an ext4 filesystem has the metadata_csum_seed feature enabled, then grub-install will not work and report this grub-install: error: unknown filesystem error."
Somehow that feature became enabled by default in recent kernels.