It looks like a soil issue to me mate, a very similar thing happened to me earlier this year. I was using B&Q tub and basket compost which I've used for years. They changed the bag and obviously the soil too cos I lost 6 plants in it. They got to the same size as yours and frazzled from the bottom up.
Ya could try potting them up into a canna specific soil like Allmix or canna terra and see how they get on.
I have heard of plants getting lumen shock from LEDs but you'd expect that to affect the top of the plant first eh?