This was just a huge, huge coincidence. An explanation on what happened with the website (I confirmed this myself back then). The banner itself is multiple images on the website, and there happens to be a split where that line is next to Isabelle. In theory, this allows the website team to only change the sections that need updating.
On that day, they uploaded the sections with new characters. They didn't immediately upload the adjacent sections that changed because of shifting characters. In addition, some users browsers may not have updated the new section images at the same time, so they saw that seam.
Now, the grinch leaker likely knew there was a seam there because they probably used the website banner images as their base. They just happened to draw isaac near one of those seams.
Tl;dr: coincidence and split website images