Apparently the issue has been fixed on the Thinkpad T420, but the T420s does not boot a GPT formatted hard disk drive (Root on ZFS).
It is a known problem that the X220 and T420 refuse to boot in legacy mode from GPT formatted disks. I suspect that the T420s is affected as well. You can either boot in legacy mode from a MBR + bsdlabel formatted disk or boot in EFI mode with the experimental UFS only EFI bootloader.
When doing the root-of-zfs install, cycle the partition type option from 'GPT' to 'GPT + LenovoFix' and see if that solves it.
If it does, attach the output of:
kenv | grep smbios
to this ticket, and I'll add your hardware to the blacklist
Alternatively, gpart set -a lenovofix on your disk.