I've tried reinstalling SuperSU. Didn't help. I have systemless root on a Moto X Pure and SuperSU 2.82.
I really don't want to leave it with grant as the default action. Not safe. I've been able to find a few other posts with a similar problem but they don't lead to solutions. This is a new behavior. Prompt used to work. About a week ago, I installed the latest version of Sensible Android Auto in Xposed. It worked twice and then stopped. My guess is that is the time when SuperSU stopped granting root. Everything that needs root can get root but ONLY if root is granted automatically to anything that asks for it.