Traditionally resort fees were not charged on reward stays. Over the last year a few people have reported being charged resort fees when on reward stays other than premium rewards. You should probably check with the property. I was not charged resort fees on a couple of reward stays in the last year.
Some elite members (mostly diamonds)have reported they have gotten properties to waive resort fees on paid stays by arguing that most of the things they are paying for by the resort fee are already benefits of status.