I used manjaro-kde-21.2.4-220301-linux515.iso to set up a new system with manual partitioning (ext4) and grub-mkconfig seems to be choking on ## BEGIN /etc/grub.d/41_snapshots-btrfs ###
Removing grub-btrfs fixes the issue.
The script should either be fault-tolerant or the package should be removed on installation on ext4.