This is the expected behavior at the moment. It may change in future.
When you open a big image (4MB+ or 4000x4000px.), it is not shown properly. The lower part of such images is white, and it's not possible to zoom in.
The lower part is white, not possible to zoom the image.
You see the full image, and you're able to zoom.
Any iOS device since October 2019 (v.5.11)