I'm having this issue, and sadly it seems like for my use case I need the newer Linux kernel (some Docker images).
Currently I'm getting the behaviour as follows.
Micro SD
Debian 4.4.y - boots
Debian 5.10.y - boots
eMMC
Debian 4.4.y - boots
Debian 5.10.y - blank screen never even seen any boot logs
I do hope this gets fixed in the future as I would like to keep using my eMMC module but I know the Armbian team doesn't get paid and sadly the Pine64 team does not do any software support (sadly I've made the mistake of buying from them twice now). When the NAS box works it is good but there is just so much trial and error to get to a working and supported Linux going on it. 😞