Traditionally the first boot track of the disk was left unpartitioned. This area is used to embed the grub2 core.img file. The size of this area used to typically be 62 sectors. In recent years the typical size has changed to 2048 sectors to keep the partitions aligned to a 1 MiB boundary for performance reasons on SSDs and newer hard disks with 4KiB sector sizes. All but the most trivial configurations of grub no longer fit in the old 62 sector size embed area. This results in grub complaining that your embed area is unusually small.
Upstream appears to have no desire to support such configurations, so this is unlikely to be fixed, but I will leave this bug report open for now. The workaround for the problem is to repartition the disk with modern partitioning tools that will align partitions to 1MiB, thus leaving 2048 sectors for the embed area.
Another workaround is to setup a simple ext4 /boot partition rather than try to boot directly from raid or lvm or btrfs.