Hi again,
So sorry to have left this hanging, due to lack of time.
I have been trying successive updates since posting this, without the issue being resolved, but have now confirmed that this is kernel or device-tree related. Using armbian-config to load kernels, I can say that the problem originated some time between kernel 6.1.104 (02/02/2025) and 6.6.750 (03/02/25). dmesg output looks similar with and without the problem, as does /proc/device-tree, but I can post any specific output here if anyone can suggest what to look for. 6.6.750 keeps the backlight and a noisy/gray screen, while the screen is black (with backlight showing in the corners) with later kernels, suggesting a further change around 6.12 (tbc)
Unfortunately I can't see where to find any armbian intermediate kernels / device-trees, so would be very happy if someone could point me at a repo. where they might be available.
Failing that I can try to locate kernels from alternative sources, or compile my own, but it would be good if I could keep as much the same as possible (including any Armbian patches and the armbian dt packages.)