There seems to be a slight mismatch, which had me beat for a while.
So in the page inspector you can add a PNG or an SVG as the page icon, and the drop well accepts either.
Top pages will display either SVG or PNG as the page icon is you choose to show icons in Top Pages.
Tree Nav will not. It only works with SVG page icons. If you have added PNG’s as your page icons it just displays the resource url.
Top pages works with either PNG or SVG, iKonic is SVG, Bevels is PNG, and it works fine.
Where as in Tree Nav only SVG works :
