Arm64 (arm in general in fact) has a rather fundamental problem with
D-I, which is that both serial and display are sensible default
devices for the installer to run on. Which is 'correct' depends very
much on the hardware and the circumstances. You may be installing a
server in rack, or a dev board with no display, in which case serial
is ideal, or you may have a chromebook or an ARM desktop machine with
a screen plugged in and no easy access to the serial console.
This problem doesn't arise on x86 where there is 'always' a screen (or
some BIOs magic to reflect what would be on the screen to serial).
Steve (McIntyre) and I have been thinking about what to do about this,
so did some investigation and came up with a plan. Essentially it was
to run d-i on both if they are configured/available. This way anyone
looking at just one or the other will see D-I as they expect. The
patch is not intrusive and essentially nothing changes if there is
only one console so this should be a low-risk change.