hello my guess is, it seems as the plant suffered from abundance of nutrients lately.that's why leaves turn out like this I think.Same happened to me a couple of times before right after switch to flower when I watered them with insufficient amounts of water,they couldn't absorb the excess nutes and this occured.I don't know how things work in dwc system but there might be something close to this.looking like phosphorus problem but again I think it is because of excess nutrients.cheers