Probably is a little late, but just in case I will leave you my answer. I had the same problem with Safari and I figured out that this seems to be a Safari issue/bug. You can work around this bug just wrapping your SVG tag with another HTML tag like a div and apply to this element the drop-shadow filter as you did in your example.
Here you have your example modified with the wrapper element
https://codepen.io/mauromjg/pen/rLaqWG
<div class="svg-wrapper">
<svg>...</svg>
</div>
//CSS
.svg-wrapper {
-webkit-filter: drop-shadow( 2px 2px 4px rgba(0,0,0,.4) );
filter: drop-shadow( 2px 2px 4px rgba(0,0,0,.4) );
}
Hope that helps!