Note that, while I haven't been able to reproduce this by booting into Ubuntu from disk, I can easily reproduce it by booting d-i from disk - specifically, the linux and initrd.gz here:
http://ports.ubuntu.com/ubuntu-ports/dists/xenial/main/installer-arm64/20101020ubuntu451/images/netboot/ubuntu-installer/arm64/
Note that, while I haven't been able to reproduce this by booting into Ubuntu from disk, I can easily reproduce it by booting d-i from disk - specifically, the linux and initrd.gz here:
http:// ports.ubuntu. com/ubuntu- ports/dists/ xenial/ main/installer- arm64/20101020u buntu451/ images/ netboot/ ubuntu- installer/ arm64/