I had a purpleheart slimline crack once. It was one of my first few pens I made and had a friction polish finish. I can't really say why it cracked. I'm not aware that it's prone to cracking from heat. One thing I did think tho...it was turned down very slim so this coulda had something to do with it.
Purpleheart is really a very stable wood. It is used for furniture, even gunstocks. Not a cracking prone wood. Problem you encountered is more likely from outside forces.
The pen was in a display case in an air conditioned store (our store) not dropped, no heat, or sun issues. It was on a European and the bottom towards the nib had a bulb shape, which is where it cracked. The pen was about two months old or more.
Anyway we were just trying to see if we can find a reason behind the cracking.