It looks to be sun burn too much direct sunlight to me. Have you try'd a diffusing sheet? would cut the amount of light they are able to get and allow you to put a fan at one end on the realy bad days and even putting the tray raised up a little to allow air flow below stopping the stone floor heating it from below.
I wouldn't worry about it my young divisions are doing the same in the north west UK they keep growing back as long as you see a new pitcher the plant is happy
pic was taken 5 mins befor posting of my young divisions well neglected