Hi Matt,
Unfortunately the solution means adding a 300 line javascript code to the theme just for IE8 placeholders – not sure how I feel about that.
If you need to support IE8, give this plugin a try: https://wordpress.org/plugins/html5-placeholder-polyfill/
It hasn’t been updated in a while, but I took a look at the code and it’s harmless.
Once installed and activated, add this code into “GP Hooks > wp_footer”:
<script>
jQuery('.search-field').placeholder();
</script>
That should do it 🙂