Alright, Paul. I have a couple of links for you to test.
I do see a slight difference in the Flash output vs. the HTML5 output of your file. Take a look at these, and let me know if that matches what you were seeing:
I also uploaded your PNG image to a new Storyline file, along with 2 other PNG images. The white line seems to be most noticeable in with your image, but I do see a very faint white line with the other images, as well.
Here's a screen recording of what I saw in Chrome v60. I'm going to take this to my team, and we'll determine next steps. Sometimes they catch errors in my logic, and we don’t have a bug after all. Of course if that’s the case, I’ll let you know!
Thanks again for bringing this up here, and I'm sorry if it's slowing you down.