Site icon not displaying properly

When using the new site icon feature, I have no problem uploading the image, and it is displayed everywhere it should be displayed, but it is not being displayed properly.

In Safari on OS X, the favicon is shown as the right half of the image, but stretched to become square. When saving the site to my home screen on my iPhone, the icon is not stretched but heavily cropped.

The uploaded image is a square png, 512×512 pixels, and the image itself displays properly in Finder, Photoshop, etc (I’ve also tried with a jpg, but same issue).

I’m using the theme Hueman, with the latest update. This theme has an option to upload a 16×16 classic favicon as well, but I’m not using this option.

Can anyone help? Does anyone have the same issue?