My guess is it was either 1) a programming error or 2) an operator error.

I was on one of the first UA 787 international flights a few years ago and there were some pretty unhappy customers when the purser found the lighting controls too complicated and just left the lights on full white the entire DEN-NRT flight. She announced to the entire plane that she had not been trained on how to use the interface and did not want to break anything.

It could also be that the "night" mode was programmed to be the blue light and not fully dark. On other planes with the fancy lighting the overnight lights are a very faint blue/purple and perhaps the pre-set is the right color but not dim enough.