Just to confirm, are the Google Material icons disappearing when the icon’s bounding rectangle’s auto-fit is disabled? This could be caused by scaling down the size of the box. There is a issue that we have on file regarding the 3.0.1 version of the Google Material Icons font where disabling auto-fit for a widget can make it so that the font icon does not render in Axure RP 8. Could you please let me know if setting the rectangle where the icon is placed to auto-fit allows the font icon to render correctly? You can set the rectangle to auto-fit to the contents of the rectangle by double-clicking one of the rectangle’s corner handles.
Additionally, if the space used for this rectangle does not match the dimensions you would like to use for your button, you could place a hotspot over the icon, so that the correct button dimensions are clickable. I hope this helps some!