I’ve seen this a few times, but can’t pinpoint a cause.
Two cases noted for certain:
Mid-treadmill run I saw a strange dashed line drawn in grey/black slightly offset from a normal data field divider (horizontal). I think I pressed Lap (or auto-lap occurred), the line went away.
Today I was running a stopwatch and noticed a line across the center. I took a photo to show it, then paused the timer and noticed the line was gone.
I suspect a bug in refreshing / clearing the screen in certain scenarios.