Invalid Safari preview with transparent borders

[phbernard]

In some cases, the Safari preview is invalid. The preview is generated when the favicon is "shared" from the result page or when using the WordPress plugin.

To reproduce the issue:

• Use the following image as the master picture:
  
• Leave everything by default (in particular, the "silhouette" mode)
• In the end, the SVG mask icon is an horizontal bar, vertically centered in the middle of the image.

The generated Safari preview is:

(why is the bar at the top of the icon area?)

Whereas it should be:

Issue initially reported here: https://wordpress.org/support/topic/safari-preview-incorrect/