Hi,
I have found why APPEND didn’t work with initrd flashed by JetPack 4.4 on Xavier AGX. In AGX thread To work, it requires init script modification in initrd.
In case initrd, created by update-initramfs, it works with just APPEND in extlinux.conf, as you reported early. But in case of Xavier AGX, there is error “xusb firmware not found” at initrd step and usb devices don’t work after boot.