This does indeed look like a bug.
Problem is that fonts need to be applied as a pseudo element, and the <button>
element doesn’t allow for that.
It may be necessary to hide the button when the font icon option is active:
.search-form .search-submit {
display: none;
}
We’ll take a look at a better solution in GP 3.1.0, although the above may be the best solution as the font icons are something we’re replacing with SVGs.