I would have thought this is virtually impossible, had I of not seen it with my own eyes.
I recently changed from centering an icon on some navigational buttons using 'Fill' fields and went the 'Icon' route, as some serious scaling issues appeared in the last couple of weeks with the new release (i.e., add an image under the 'fill' section, "normal" image fit, still scaled and blown up like I used "fill" image fit; this also broke all onHover and other effects, too).
So I make the change to my buttons, supplanted the 'fill' icons with the icons using the, well, 'icon' fields and the world was good again...or so I thought.
Buttons look great in my local pbix file; they look great when viewing in PowerBi Servives dashboard; shared with collegues and the buttons looked great; however, the moment I published to the web and wrapped the dashboard in a Drupal iFrame, I lost the icons!!!!
All functionality is there...except no buttons. Unexplainable and unsure how to address at my level.
I am going back to change a couple to use the (albeit now JUMBO) 'fill' method to center a 50px icon in the center of the button just to ensure they compile in the iFrame, but every single button icon is missing in the iFrame (though, again, looks perfect in the source PowerBI Services location)...impossible, I thought, given it's merely an iFrame.