Why do several of my leaves look like this ?

Hmm honestly good on you to notice that and I think it is a problem not to be overlooked like another person responded. One of the telltale signs of a problem is the red stems. It also kind of looks like light burn. I checked your pictures your lights distance looks fine. The thing about UV lights is people move them close because they run cool but they can still burn plants kind of like how a snowboarder can still get sun burn in the cold. I think you are okay for lights but this is a possibility. The other thing I was thinking was nitrogen toxicity/deficiency but there are some signs on your pics that this is not the case. There isn't enough yellowing for deficiency and it isn't dark enough or are the leaves curled down for toxicity. These are still small things to look at but probably not the root cause. By the looks of it, you have the beginning of a phosphorus deficiency. The images you have shown do match what a phosphorus deficiency looks like. A cannabis phosphorus deficiency generally appears on leaves from the lower/older parts of the plant. The lower leaves may turn dark green or yellow (similar to nitrogen toxicity/deficiency which is why I mentioned it earlier), and start getting spots or big splotches that look brown, bronze or even a little blue. The leaves may thicken and curl, and the affected leaves feel stiff. Sometimes the stems of the plant turn bright red or purple, but not always. Sometimes accompanied by a calcium deficiency, as phosphorus and calcium interact with each other in the plant Do not mistake natural reddish-purple colored stems for a phosphorous deficiency!