Genetics. Plenty of genetics will rot at 70% RH. Some won’t. Never make a blanket statement about cannabis.

65% is high enough for mold and mildew to form but it’s mainly a concern with large dense flowers. Huge colas can have a micro-climate inside. You really want to stay under 60% RH with good air flow in flower to ensure nothing takes hold.

This leaf is not exhibiting a magnesium issue. A leaf does not tell a story. We need to see the whole plant to give advice. Where ‘damaged’ leaves are on the plant matters.