Note: This is the iOS 13 beta, but also could apply to the official release tomorrow.
Update 2: I replaced it with a larger thumb
I tried to find something during the last hour. It looks like the swipe gesture recognizer as interferance with UISlider.
The problem appears only if you are using right/left direction. Perhaps you should wait before update your app or change your UI to use the up/down swipe gesture.
At this time the slider works normally if you wait a bit before you move your finger. Hoping Apple will fix it quickly.