That’s pretty strange.
I’m assuming that you are talking about the 2018 iPad pro-11-inch model?
I only have an old regular iPad 5th generation. The 11-inch iPad list 2388-by-1668-pixels screen size. That should equal a logical screen width of 1334 pixels in portrait mode.
I can display the page fine with the width set to 1334 in the developer tools. There should be nothing about a site image that would affect the way the image responses to an iPad.
Does this happen if you lost the page in portrait mode or if you rotate the iPad after you have the page loaded? Does reloading the page help?