Mine has the same problem. I also walked to the tmo store and checked showing s6 and s6 edge, both have the same problem. The max brightness cannot be set in s6 and s6 edge. Many people are discussing this in the different topics too.
When you test your phone, please try to unlock -> lock -> unlock -> lock the phone during you push the flash light direct to the ligh censer, you will reproduce this problem more easily (waiting 5-10 sec for each unlock and lock). Also, you have to set the brightnes level to the max with out auto check box in order to see this problem.
Hope this help And hope they will fix this soon.