I'm not sure how often this happens... but it also happened on an A380 from HKG to LHR at then end of the august. I'm not sure why they didn't do it, I was kinda thinking "well if they oversight this security issue, how do they check other things?". It is important that cabin light dim during the ours of darkness (you need to accommodate to a lower light intensity in case of an emergency) - however I didn't ask why they didn't do it.