I recently tried this as well. And ran into the same issues. I did eventually manage to resolve this by flashing a manjaro build to an sdcard. Opening up the pinebook pro. Toggling the eMMC off using the little switch. Starting the laptop. Pressing any key while it was in uboot so it would just pause (I am really not sure if all the manjaro images have this, in either case it was Manjaro-ARM-sway-22.04.img). Toggling the eMMC back on, which would now be detected uboot and later on Linux itself.
After this I continued on to install Manjaro because I was just really annoyed at that moment, but I would assume that manually flashing a working uboot at this point would work. Hope this is useful to anyone. And hope this is getting fixed at some point.
