Are you sure you're watering with the right frequency? It looks like overwatering; the leaves can look like that when the cells have too much water and swell, which is reflected in those initial symptoms. 😊
maybe a mutation? unless you think it is re-veging, but that looks a bit different. (can see example of revege in my gp-redux diary.)
google "marijuana mutation" maybe add smooth leaves or non-serrated if not immediate answer near top.