What's is this never seen anything like this ...

I have had similar spots if i'm adding nutes and splash a drop on a leaf. Doesn't happen with plain water, just with nutes. I've had them burn a hole clear through a leaf. Yeah, i'm a new grower. Lol... Good luck.

Do you smoke cigarettes? (TMV) Not even sure if tobacco mosaic virus can infect a marijuana plant for sure? nor, if that type of secondary contact can transfer it? Ensure substrate is getting slightly dry before irrigating, as that looks like something caused in the substrate, unless you often find standing water on the leaf in that location very often. Scope underneath leaves for any tiny pests. It's not nutrient related that i can tell, so substrate or pest related. not much else to choose from... hopefully someone can add more.