Hmm.. Maybe I said spoke too soon..
What I see is that if I build an image from the Armbian tree like this
./compile.sh BOARD=rock-5b BRANCH=legacy RELEASE=jammy COMPRESS_OUTPUTIMAGE=img KERNEL_CONFIGURE=no BUILD_MINIMAL=no BUILD_DESKTOP=no
and boot from it my older (and problematic) screen works ok on HDMI-2 but not on HDMI-1 at the initial boot.
However, if I do an apt update / apt upgrade and reboot my problematic screen stops working again...
This seems not-so-good...
Any insights on why this can happen?