Hmm - so now I have a problem 😩 I'd checked to see whether I was on firmware v2.0,5 or v2.1.0 (... which is arguably less obvious than it should be), and thought I was on v2.1.0. But I booted from eMMC (with an SD card present) by interrupting the boot process, and doing:

devnum=0
devtype=ubi
ubi part spi-nand0
run distro_bootcmd

... and from the system that booted (successfully) as a result, I got:

# tpi info
|---key----|-----------value------------|
 api       : 1.1
 build_version: 2023.08
 buildroot : "Buildroot 2023.08"
 buildtime : 2023-11-28 14:01:07-00:00
 ip        : Unknown
 mac       : Unknown
 version   : 2.0.5
|----------|----------------------------|

... so I decided, possibly somewhat cavalierly, to do an OTA upgrade to v2.1.0.