Visually complete inconsistent

[mmmoth]

We're getting inconsistent results with visually complete. The majority of our runs we get a visually complete score of around 2.1s. But we get some runs where visually complete is 10+ seconds. Here is an example http://www.webpagetest.org/result/180618...9fa5074a/.

For the runs that take 10+ seconds, we see that it's 99% complete around 2 seconds and then several seconds later, it becomes 100%. Looking at the filmstrip and video, there's no visual change at all. We're having a tough time figuring out what the issue is here.