I'm thinking it's the heat mat toasting them. Maybe there was an instance where the water may have dried up before you replenished it? Or maybe its just getting too hot? the curling in of the fan leaf is making me think it's caused by heat.
Just taking a guess. I've cloned a very similar way.. it looks like they are in a cloner? I had mine in a small tray with dome, and sat the tray on top of a plant saucer to give it space, and to not warm the water up too much.
Edit: I know the temp isn't high on the hygrometer, but I assumed it's lower due to the tent being open.