Boot fail: 12.4-R-{dvd1,disc1}.iso via USB

I downloaded dvd1.iso just after its release and dd'd it to my 8G stick, previously 12.3-R dvd1, using standard arguments: bs=1m conv=sync

I'd been testing my bsdconfig patches successfully with this, the image works fine loaded as cd9660 on the Thinkpad T430s.  I was just about to delete my freebsd slice prior to adding 16GiB to it and reinstalling 12.4 from scratch from that stick ...

So I was horrified when that same stick failed to boot my Thinkpad X200, saying only (from a photo):

_____________
CD Loader 1.2

Building the boot loader arguments
Looking up /BOOT/LOADER... File not found
Looking up /boot/loader... File not found
Boot failed
____________

After much checking of X200 BIOS settings, then dd'ing the stick back to the T430s, diff and cmp both showing it as identical, I tried booting it on the T430s -- same 'Boot failed' as the above!

So then I made a stick from the disc1 .iso and tried that -- same failure!

Yes, I'd tested sha256s on both the downloaded .xz and decompressed .iso files, all good.

Can I really be the only one who's tried booting these images via USB?

Or is this a problem only happening on some older Lenovo Thinkpads?

Meanwhile I've restored 12.3-R dvd1 to the 8G stick; still works of course.

References:

FreeBSD-12.4-RELEASE-amd64-dvd1.iso
FreeBSD-12.4-RELEASE-amd64-disc1.iso
https://www.freebsd.org/releases/12.4R/announce/
https://www.freebsd.org/releases/12.4R/errata/
(none so far)