Hi,

Other than the bit of cupping or ruching I can make out, the leaves and stem look otherwise healthy and lack the signs of other issues like leaf miners or other pests. You checked leaf undersides for aphids?  If there was a nutrient issue you might see yellowing, ior edge scorch or yellowing between veins which is not seen here. 

If you regularly rotate where you grow it and treat it as a heavy feeder plants needed compost/balanced fertilizer regularly, I'm going to assume this is an environmental issue caused by the temperature swings we have been having or a variety with thicker leaves that can tend to this trait. 

Sue

I can't enlarge the photo enough to see if there is anything unusual. I am not finding any of the usual symptoms of disease or of insect damage that leads to an obvious conclusion. Chard is usually pretty  free of most problems. Does the texture of the leaf surface feel different in some way when compared to others? If you use a magnifier on your phone do look closely do you see anything that would be a better indicator than the photos we have show? Right now, I am not coming up with a reason beyond environmental reasons or very rich soil that led to fast growth and maybe some malformation.

I did see a photo of Bright Lights chard and a couple of the green leaves were very distorted, but OK enough to be used in an advertisement.