When pistills turn brown it's because of the oxidization process from contact with the air. So basically the pistills have an outter membrane. When this membrane is pierced then the pistills turn brown. The membrane can degrade and pierce from many things. First, some phenotypes are just like that so that could be why. Second, it could be wind, air or contact. Third it could be light or humidity. Actually it doesnt matter at all for the final product, after all the pistills will all turn brown after the harvest anyway. Hope this helps ! 🚀