Dear @SteveW
Please verify whether the backlight is on. The backlight electronics and -software are independent of the actual picture display. On most displays you can see the light on the edge of the screen, even if a black image is shown.
My guess is, that the ss.bl_gpio was not explicitly configured to the correct values, and after the change of the bootloader version, the backlight control pin floats to the opposite level.
Regards, Andy