ThinkPad X260 won't boot 14.3-RELEASE installation media

I have a ThinkPad X260 that's already running FreeBSD 13 successfully
but want to do a fresh install of 14.3-R. It consistently refuses to
boot from an installation memory stick. What happens is that I select
the USB stick in the F12 boot select menu, the screen goes black for a
while, and then the machine prompts me for the geli decryption key for
the onboard disc and boots into the already installed 13-R system.

I have tried:

a) using a USB 3.2 memory stick, a USB 2.0 memory stick and even a micro
SD card in a USB adapter

and

b) using both memstick.img and disc1.iso images on all of the above

and

c) inserting the memory stick in every USB port on the X260.

None of these combinations will boot on the X260. All of them boot
successfully on my wife's ThinkPad T460s. They also boot successfully on
a cheap Shenzhen cube I've got lying around.

For completeness, to save people asking the obvious, both ThinkPads are
already running non-Windows OSes with Secure Boot off, pure UEFI boot
with CSM enabled on both. This is the amd64 version of FBSD, downloaded
this morning from https://download.freebsd.org/releases/ISO-IMAGES/14.3/.

Does anyone know of any hardware gotchas or other problems that might be
biting me here?

-- 
All network cabling aspires to the condition of macramé.