My amateur guess would be Magnesium deficiency. most likely due to the Coco coir not being buffered. I think Canna Coco A and B contains enoughf Magnesium but it's designed to be used with tap water at around 0.3-0.5EC.
If you are using RO or a lower EC tap water you should add some Cal/Mag.
I don't Know if you can buffer the Coco coir after you started using it already but I think that is the problem.
Hopefully someone with more experience can correct me if I'm wrong