I used to use an iPad mini 4 as the home hub. In iOS 11 beta sometimes my iPhone7P (that is also on iOS 11 beta) couldn’t access the HomeKit devices and I couldn’t figure out why. My other older devices NOT on beta are fine. As a last ditch try, I disabled the home hub function on my iOS 11 iPad, having NO home hub right now, and now all my iOS 11 beta devices can access the HomeKit devices more consistently.