Happens on my endurance bike as well, since it has an 11-32t cassette and the R8000 SS cage can't quite pick up the full slack on the 13t cog when in the small 34t chainring. (Di2 won't cross chain to the 11t or 12t, but that's based on telling it it's an 11-30 cassette; the 13t should technically probably not be used, but it has no setting for this cassette.) If I pinch one chain link I can feel the chain tug against the cage spring, so it would probably go away with an 11-30t cassette; it's just the very end where the chain gets untensioned. And in fact I don't get it on my other bike with 52/36 and 11-30. I suspect this is why Shimano doesn't spec the SS R8000 RD for use with an 11-32t cassette, since you would lose the 13t, or the big-big. Stick a bit of 3M clear protective film on the chainstay though, and it's a non-issue.