Regarding this issue:
sudo sed 's,FDTDIR \([^ ]*\),FDT \1/rockchip/rk3399-pinebook-pro.dtb,'
I only ran into this issue with a few newer Pinebook Pro’s
It turns out the stock Debian’s u-boot on the emmc is causing the boot process to look for rockchip-evb_rk3399.dtb.
For example if you install the Manjaro u-boot files to the emmc, this problem goes away.
That’s why only some of us encounter this problem and others do not.