You nearly hit it. it looks cleary like a calcium defiency. Most time caused due the rootzone, the nutrient cannot be absorbed by the plant. Could be caused by Ph issue, or your nutrients are not cannabisfriendly enough. Solutions: check your Ph ( runoff ) , if its wrong give a nice flush and soil will stabilize.
Give the plant additive calcium after stabilizing the PH. the damaged leaves wont recover, check the new growing leaves, if it happens again