Failed to install bootblocks. You will not be able to boot OpenBSD

Hello OpenBSD Community,

Hope you all are staying safe during these crazy times.

I am looking for any feedback on an installation error that occurred using
the custom-layout partition option across two SSDs.

ERROR:

Installboot: no OpenBSD partition
Failed to install bootblocks.
You will not be able to boot OpenBSD from sd0

VERSION:

OpenBSD 6.6 release/install66.fs media

MACHINE ARCHITECTURE:

amd64/AMD Ryzen 5 chipset

BACKGROUND:

The plan was to install OpenBSD 6.6 across two disks. Previously, these
disks had FreeBSD-12.1-ZFS installed on them. Since the disks were new and
had no data on them, other than the FreeBSD installation sets, I decided
not to clean the boot code area with 'dd if=/dev/zero of=/dev/rsd0c bs=1
count=1'.

INSTALLATION STEPS:

1) Initialized disks for a GPT schema:

# fdisk -iy -g -b 960 sd0
# fdisk -iy -g -b 960 sd1

2) Entered the installer, choosing the custom-layout option for a whole
disk GPT
3) Cleared the auto-generated partitions, and created all new partitions
across sd0 and sd1
4) At the error installer dropped into a shell. At the shell, I entered
reboot, and the machine booted.
5) Logged into the machine and ran the installboot command:

$ doas installboot -nv sd0

Output:

Using / as root
would install bootstrap on /dev/rsd0c
using first-stage /usr/mdec/biosboot, second-stage /usr/mdec/boot
would copy /usr/mdec/boot to //boot
looking for superblock at 65536
bad superblock magic 0x0
lookign for superblock at 8192
found valid ffs1 superblock
//boot is 6 blocks x 16384 bytes
fs block shift 2; part offset 1024; inode block 24, offset 1704
expecting 32-bit fs blocks (incr 0)
master boot record (MBR) at secto 0
partition 0: type 0xEE offset 1 size 4294967295
installboot: no OpenBSD partition

KEY OBSERVATIONS:

1) The error only occurs with the custom-layout option. When OpenBSD is
installed on a single disk using the auto-layout option, the error does not
occur

2) The error says there is "no OpenBSD partition," but there is an
OpenBSD partition.

$ doas fdisk sd0

Output:

Disk: sd0 Usable LBA: 64 to 976772081 [976772081 Sectors]
#: type [ start: size ]

------------------------------------------------------------------------------------
1: EFI Sys [ 64: 960 ]
2: OpenBSD [ 1024: 976772081 ]

3) The machine seems to boot and run fine.

$ doas reboot

Output:

probing: pc0 mem[640K 63M 92M 16M 3308M 1M 42M 29171M]
disk: hd0 hd1
>> OpenBSD/amd64 BOOTX64 3.46
boot>
booting hd0a:/bsd: 12858696+2749448+326464+0+704512
[806406+128+1021271]

4) The system successfully updates to current - it generates the error -
but it updates and reboots on its own.

5) The 'installboot' command generates a "bad superblock magic 0x0" error

QUESTIONS:

Why does the error say that there is no OpenBSD partition?
Why does the error only occur with the custom-layout option?
Should I have cleaned the boot-code region with dd if=/dev/zero
of=/dev/rsd0c bs=1 count=1 before the installation?
Is the "bad superblock magic 0x0 error" related to pre-existing garabage
in the boot-code region?