If the machine has no monitor attached at all before boot, KMS couldn't probe
the monitor and make it goes into native resolution afterwards. This issue is
not very obvious when DVI/VGA are the only connectors available because of
their mechanism.
When it comes to HDMI/DP, the monitor is considered disconnected when it's
powered off. Then the issue become annoying, while we have to make sure that
one monitor is connected and switched on before boot.
Moreover, display managers have to be started manually under such situation
after you turn on your monitor. Not even all DM can start X in optimal
resolution without some "xrandr hack"
By the way, it's funny that if you got one attached, you can power on others
later without getting into the problem. Just that I am not sure if it's because
all my monitors have the same optimal resolution.