That's really bizarre. I take it that you can replicate it each time you try it?
Since the page starts with the last word already on the right, does that mean that you are scrolling off to the left, than back to the right (and that's when it disappears)? If that's the case, can you move just a little to the left and then back to the right, or do you have to move a significant amount?
By chance, if you can get a video of it happening (from when you enter the page to when it happens), I'd *love* to see it. You can send videos directly to me at firstname.lastname@example.org!