So, whatever HP did in the UEFI update "unfucked" the relevant bits and freed the offset that OpenBSD uses.
And now that I am sharing this - I recall that I have experienced the same before - on a HP Elitedesk (IIRC: DM 800 G5).
I hope that this might be useful information for others. If you have some HP hardware that won't boot OpenBSD, make sure you have the latest firmware.
I don't know if this affects the Elite series only or also the Pro series.





