Well this is odd. I've had nothing further from YF, so I thought I'd try my Fritz Box 7530 (which had been happily working with IPv6 from AAISP for years). It took a while, but I got it to connect and it clearly shows my IPv6 addresses. However, the various checkers I use still say v6 undetected.
As a further complication, I went back the eero, and now I can't get that to work. ONT shows normal LEDs, but the eero blinks white for ages before turning red. Both ONT and eero have had several reboots.
Currently working happily using the Fritz Box (which has always been my preferred option), but confused about the lack of IPv6 visibility.