I mentioned this to Charlotte awhile ago and she asked me to video it. It's not a big deal, but it is strange. This was made in mid June on a Samsung Nexus 5 or 6 with the then-latest version of the app.
I used slow-mo video on the second half of the video (in case the screen refresh * recorded fps were causing the effect to go by too quickly for you to diagnose). So the effect on the second two times is very exaggerated.