I think it's a nitrogen toxicity in combination with a high ec value. Then you have 'the claw', this is a real nitrogen toxicity indicator. Also the leaves get crispy. Especially a sativa can have this, indica loves more nitrogen. Liberty haze tends to have this when you change the nutes too late to bloom in week 3/4.
So just give more pk from week 5/6. And some extra trace elements with it because you give less bloom solution. Def not a cal/mag deficiency, then your leaves start to yellow and drop op instead of crumbling before dropping. Also leaves will get droopy.
In week 7/8 it's normal for the big leaves to yellow and crumble because you have to gradually reduce the nitrogen the more you get into bloom phase.