Yes, the seam is very unfortunate!
The size requirement didn't really change but the format did in that the ATP logo occupied about 300 pixels on the left and pushed the photo over to the right. Now the photo goes the length of the banner and the logo sits on top of it.
On a small screen or a phone, it doesn't repeat, but on a wide screen it does. I have a wide monitor too, and I have always kept my window size smaller for this site to avoid the seam in the banners. If I open it full-wide, for example, I get the split in the middle and two bees
I think many of the potential banners in the queue might be short for this format because the minimum length has been 1200 pixels; and also, the 'area of interest' has tended to be the first portion of the photo, so many of the potential banners may do better being flipped to accommodate the logo. It is my intention to make some 'new and improved' ones, longer and better for the NGA, but I haven't had much time lately to play with pictures with this new goal in mind...