This has been solved via the addition of the following CSS:
.ui-slider-default ui-slider-thumb,
.ui-slider-default [ui-slider-thumb],
.ui-slider-default .ui-slider-thumb {
z-index: 1;
}
ui-slider:after, [ui-slider]:after, .ui-slider:after {
z-index: 0;
}
See plunker: http://plnkr.co/edit/HPOAq0oV6GnE0JevuZI4?p=preview