Something broke. The VPS would not boot:

 ZFS: out of temporary buffer space

So this sounds like a missing step in the automated upgrade flow. Normally, using new features in zpool is deferred until you choose to upgrade the pool after the reboot, so you get to see the warnings. At a guess (because I'm on FreeBSD 11.4 still), the OpenZFS migration forces the issue to do the zpool upgrade early and they missed the gpart requirement. //

boot from the current rescue disk
bring ifaces up
scp a current/13 zfsbootcode file
install that
//

gpart bootcode -p /root/Downloads/gptzfsboot -i<gpart index of freebsd-boot> <block device>
with that just use the correct path from gptzfsboot
or just dd if=/root/Downloads/gptzfsboot of=/dev/vtbd0p1 if you are brave