ZFS BE not booting

returning FreeBSD user here, using bectl for the 1st time on a thinkpad x230 with 16GB of RAM.

bectl list -aDs
BE/Dataset/Snapshot                          Active Mountpoint Space Created

r359556
  zroot/ROOT/r359556                         NR     /          12.2G 2020-04-03 20:05
  r359556_at_2020-05-22-22:20:00-0              -      -          61.1M 2020-05-22 22:20

r360723
  zroot/ROOT/r360723                         -      -          6.25G 2020-05-22 22:20
    zroot/ROOT/r359556_at_2020-05-22-22:20:00-0 -      -          61.1M 2020-05-22 22:20

I created r360723 using bectl whilst being on r359556 and am still using base ZFS.

I'm running GENERIC-NODEBUG on this thinkpad x230.

[~] uname -v
FreeBSD 13.0-CURRENT #2 r359556: Mon Apr 13 13:35:31 UTC 2020     root_at_harbinger:/usr/obj/usr/src/amd64.amd64/sys/GENERIC-NODEBUG

I activate and boot into the newly created BE, run svn up -r 360723 from /usr/src and doing the upgrade steps (same steps were used in previous upgrades leading to r359556 without the use of BEs).

make -DMALLOC_PRODUCTION -j4 buildworld
make -DMALLOC_PRODUCTION -j4 buildkernel KERNCONF=GENERIC-NODEBUG
make -DMALLOC_PRODUCTION -j4 installkernel KERNCONF=GENERIC-NODEBUG

Then I reboot but the boot process drops me into the debugger (unfort. I don't have the capture of the reason).
I've tried upgrading to both r360723 and r361307 (I pick those revisions from the released snapshots, no other reason) but both produce the same result.

Loading kernel.old from the r360723 BE works fine.

Is there something I'm perhaps missing regarding loader.conf and/or rc.conf ?
It could just be these 2 specific revisions and will try another revision but perhaps I need to do something setting the zfs bootcode again using gpart ?

cheers

-- 
Marco van Lienen -- Unix SysAdmin -- https://lordsith.net/
https://keybase.io/scarcry , GnuPG id: 8580E6CB
"The Tuck Pendleton machine...zero defects."