Rust marks on young Lemon Cherry Cookie Auto (2 others next to her are fine)

I would just cut the leaves off and don't mind 😝 They are hindering the air flow anyway 😎 Perhaps the old leaves dont want to exist longer when they dont see light and air 👻

Hard to say, so it could be that the humidifier might harm them if it's constantly damp, as it encourages mold growth. But it could also be signs of CalMag deficiency. Try moving the humidifier a bit further away if possible. And keep checking. Add some CalMag, that's always good for prevention; I add 0.5 ml per liter with every watering just to be safe. Good luck!