Ok, I'm not crazy, but this happened again last night, which then led me to do some experiments. This happens on my two iPhones (6S and iPhone X).
Turning the screen dimming down (say 10%), going to another app or two, then coming back to my game, I was able to observe this, which I then profiled in scout. Every other frame had the RenderUtil.clear issue doubling the frame time, giving it the choppy appearance.
When I went to make the screen brighter, the issue went away. I tested this multiple times and I'm able to repeat it. But I can't sort out why this happens or see any way around it.