Hey there my friend, pistils turn brown when they oxidize that means basically that they get damaged somehow and the oxygen in the air makes the juices inside the pistils to rust basically, that's where the color comes from. This is important to remember because it can give us clues on what to look for to answer your question. First of let me just say some genetics turn brown early (like my auto colorado cookies) and some phenos turn more quickly than others. Beyond that oxydisation of the pistils can be from any number of factors from a fly piercing the membrance so kinetic reasons, or from environmental reasons like too much airflow on that part of the plant or a too high temperature which causes the pistils to degrade. At your temperature you should have probably lower humidity or at that humidity you should keep a cooler temperature. In my opinion it's a combination of genetics and environment and it's absolutely nothing to worry about. All the pistils turn brown after the chop anyway right? Hope this helps ! π