Oh the irony!
I've owned this phone (Mi8) for 2 years, and I've been using the EU ROM since the bootloader was unlocked. I've only ever had one issue with an update, and that was due to a corrupted download, but today, on the final weekly release for the Mi8, my phone rebooted into Fastboot, and TWRP, although it did eventually load, was completely unresponsive. I tried to reflash TWRP, which was apparently successful, but any attempt to boot to recovery resulted in the phone rebooting to Fastboot, so I've just had to wipe the phone and start from scratch with a Fastboot image.
Was this a coincidence, or a sign, or something else?