non-functional /usr/mdec/boot? - NetBSD 9_Stable (20210114)

[yancm%sdf.org@localhost]

Upon a recent update of my amd64 system from 8_Stable to 9_Stable,
I updated /boot from /usr/mdec/boot and installbooted both members
of my raid set...my adventures from a year ago are here:
http://netbsd.2816.n7.nabble.com/installboot-problems-with-raidframe-amd64-tt412138.html

Upon reboot the system hangs with:
 NetBSD mbr boot

 NetBSD/x86 ffsv1 Primary bootstrap

[hangs > 2 min]
If I boot from a NetBSD 8 install cd, I can drop to the
boot prompt, see my raid set and boot back into the system...
eg
> dev
disk hd0 size 149 GB
  hd0a (RAID)
[+ other raid disk and cd and floppies...]
> boot hd0:netbsd

This put me right back in my normally functioning system...

In the thread above, it had been suggested to boot the \boot
program directly as a test...I tried that and got:

> boot hd0:boot
boot: hd0a:boot Input/output error

By putting my backup /boot from my previous 8_Stable, the machine
boots fine again, no cd required.

Could I have goofed up the installboot step? filesystem IS ffsv1
and I used the ffsv1 1st stage.

For my understanding, does the first stage loader
/usr/mdec/bootxx_ffsv1 in my case have to be the same revision as /boot?
(ie. both NetBSD 9) Could having them out-of-sync cause this?

Out an abundance of caution, I rebuilt the system again once I upgraded
and found no change in behavior (or bootloader file sizes either).

thanks in advance,
gene