On the working system, look at the device(s) in sysfs, and their
device symlink. This points to the parent device - which may in turn have its own parent device, and so on. Write yourself a list of the device and all its parent devices. Then you can check all of them in the initramfs. You might be missing more requirements than just the two disk devices.
Secondly, when you make your list of devices, look at the
driver/module for each one and write down what it is. This tells you which kernel module is recognizing the device.
udev is supposed to be loading the kernel modules for you.
Unfortunately, the LFS initramfs takes systemd-udev and tries to run it without systemd. This is unfortunate because using systemd would let systemd-udev log any errors it encountered to the systemd journal. You could then check the journal for errors. I do not know whether udev error logging works in the LFS initramfs.
does ubuntu add-on to the kernel (/ubuntu directory) dictates any special policy for initrd?