.single-scroll-down-animation {
  width: calc(25% - 20px);
  margin-bottom: -10px;
}

.scroll-down1 {
  width: 50px;
  height: 50px;
  border-radius: 100%;
  border: 2px solid #fff;
  font-size: 30px;
  overflow: hidden;
  cursor: pointer;
  display: flex;
  flex-flow: column;
  align-items: center;
  justify-content: center;
  position: relative; /* 擬似要素の基準位置に */
}

.scroll-down1::before {
  content: "";
  position: absolute;
  width: 15px;
  height: 15px;
  border-left: 2px solid #fff;
  border-bottom: 2px solid #fff;
  transform: rotate(-45deg);
  top: 0; /* 追加 */
  animation: scrollDown1 1.5s ease infinite;
}

@keyframes scrollDown1 {
  0% {
    top: 0; /* transformではなくtopで制御 */
    opacity: 0;
  }
  50% {
    top: 15px;
    opacity: 1;
  }
  100% {
    top: 30px;
    opacity: 0;
  }
}