That is an interesting bug indeed because I have that every second flight 
for no reason.
When a flight is loaded at nighttime, the cabin light is always on when viewed from the outside. When a flight is loaded during daytime, the cabin light seems to be off from the outside view and the cabin becomes fully dark at night.
When starting in the late afternoon and landing in the evening, the cabin light stays dark.
But there is also a second bug - the light that is illuminating the tail fin logo does not come on either, so the cabin light and the tail fin logo light stays off no matter if I switch it on in the EFB, or off.
This also seem to happen every second flight or so that the tail logo light just cannot be activated.
I think the cabin light is also deactivated from the outside view when deactivating the virtual cabin. (but not sure about that, I fly always only cargo only most of the time with that Airbus and therefore have the cabin light always off).
Cabin light off is better and more realistic inside the cockpit at night because there is always some annoying unrealistic light-bleed shining right through the cockpit door and into the rear parts of the cockpit, and I like my cockpit fully 100% dark with only dimmed LCD screens and dimmed background light bulbs on (INTEGR LIGHT).