Hello,
Using Armbian 23.11.1, I managed to upgrade to 25.5.2 with the hdmi output still working by pinning the version of linux-image-current-sunxi. So right now I have armbian 25.5.2 running with the kernel from 23.11.1 (6.1.63) and everything seems fine so far.
I'll see if I can bisect the linux-image versions to find which one introduced the bug.
Also note that apart from the hdmi output not working everything seems ok as I can ssh the cubieboard and use it as a headless machine without any issue.