I read the log again. So it is not cboot reads the extlinux.conf on the nvme. It tries to read, but fails. Then it uses kernel from partition. And never cares about the extlinux.conf anymore. That kernel is also from kernel partition but not APP.
So kernel cmdline is from cbootargs. But rootfs A/B expects the UUID from extlinux.conf so it is broken.
Looks like the situation is like that.