I made a custom running workout for which the last step was 'cool down until lap button pressed'. When I was at this last step the lap button (bottom right) didn't do anything. I tried pressing it multiple times, but it didn't respond. Touch was on and also didn't want to do anything either when touching the marker at the lap button. I had to pause and save the activity.
I don't think it's a hardware issue because the button works fine now after the workout and worked for the starting warm up step.