Doesn’t that mean that the GRUB should update the grubenv on every boot to record which entry you did choose?
Yes, I think it should, and the saved entry in grubenv is the correct one (i.e. the one I selected last), but it still doesn’t select that one by default next time I boot. And grubby correctly reports the kernel I want to boot with as the default. But the grub menu doesn’t actually have that one highlighted by default.