I installed the official Ubuntu Focal image from https://wiki.radxa.com/Zero/downloads to EMMC, it was booting fine but stopped working after apt-get update && apt-get upgrade (just gives a black screen without any messages).
Is it a known issue and if so is there a workaround/solution for that?
tl;dr; Official Armbian builds comes from here and builds are digitally signed. Unofficial Armbian builds are ofc not supported by Armbian so its down to Radxa to fix the problems.
Workaround - search this forum. Solution - official Armbian support.