Sorry to bring this thread back to life but I've run into the same issue. I use an HDMI switch for my several computers and of course only Tinkerboard refuses to come back. As krpt mentions above, switching between text consoles gets text video back but not the X display. I'm unsure what to do next. Do we need to implement the rules and script described above even if not rolling our own kernel? Is this documented as bug/enhancement for a future drop of Armbian?
[Update] I learned to read! followed the instructions from botfap and it's worked great. Happy to return it's working very well at 4K with a Samsung 28" monitor. Seems native resolution to me, so great! BTW this is with legacy kernel.