Makes sense that it looks like a magnesium deficiency. I would assume you had enough Mg in the soil or water, it's just that the plant couldn't uptake it properly because of low humidity. Is your house extra dry, or do you live in a dry region? I'm fairly dry here, but so far haven't had any problems like this. I just grow right out in the dry living room air and they're happy. Man there's so much still to learn.