This is one of those things that gets forgotten from time to time. There are lots of these little tasks that fall through cracks.
Often this happens because the task is not assigned to one specific person. If someone happens to remember than it gets done. And if not, the conversation goes something like this: “Oooh, the light is still on. YOU forgot to shut it.” …. “And YOU didn’t shut it either.”
To prevent that, you should jointly assign even these tiny tasks to one person. Not “Someone”. A specific person.
That person owns the responsibility, and can do it how and when they’d like.
(You might bundle several such tiny tasks into one. For example, right before Shabbos make sure that bathroom lights are on, refrigerator light is off, the crockpot is plugged in and on the right setting, there is hot water, and whatever else you need before Shabbos. Make an actual checklist – on paper. This is helpful for the person with the responsibility, and even more helpful when someone else has to take over that responsibility.)