Bootstrap Card Background Image Bug

Test

HTML

<div class="card"></div>

CSS

.card {
  position: relative;
  border-radius: 5px;/* necessary to trigger the bug +*/
  width: 100px;
  height: 100px;
  overflow: hidden;/* necessary to trigger the bug +*/
}

.card::before {
  content: ' ';
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  background-repeat: no-repeat;
  background-position: center;
  background-size: cover;
  background-image: url('https://upload.wikimedia.org/wikipedia/commons/thumb/6/64/Pentagram4.svg/240px-Pentagram4.svg.png');
  -webkit-filter: blur(5px);
     -moz-filter: blur(5px);
       -o-filter: blur(5px);
      -ms-filter: blur(5px);
          filter: blur(5px);
}