It's not Iron deficiency.

I'm not saying it's definitely the case, but I've had a similar thing happen to a various types of Asian greens (all Brassicas) when grown in my current system and I believe it may be fungal root disease.

Initially I was able to grow them in the system, but after a couple of months the lower leaves on the plants, both established and new seedlings, started developing transparent patches between the veins. Any seeds I sowed directly into the GB either had a very poor germination rate, or died within a week or two of germinating. All other plants are growing beautifully, so I've simply given up on the Asian greens in this system and I'm growing them in the veg patch instead.

Asian greens from the same punnets of seedlings and/or packets of seeds still grow wonderfully in my kids little blue barrel system, so it's not the plants, there is something in my IBC system that kills Brassicas, so I simply grow them elsewhere. Being indoors I think the IBC system may not be exposed to enough of mother natures checks & balances, whereas the kids blue barrel system is outdoors and exposed to a wide variety of natural fungi and other natural beneficial organisms... that's my theory anyway.

P.S. Looking at your pics, it definitely isn't Iron deficiency.