The Official Armbian Stretch is not working on my side either.
For info: I think I used a different way to flash u-boot than you:
- You did it via rkdeveloptool (I think?) according: https://wiki.radxa.com/Rockpi4/dev/spi-install
- And I did it via uSD card according: SPI + NVMe booting(beta)
So two ways to update the u-boot which results the same: unable to boot Official Armbian 