What is the white line inside the border button? And how do you remove it?

Question

css

.btn {
  padding: 0;
  margin: 0;

  font-size: 18px;
  font-family: $font-Inter;
  color: $primary-black;

  border: 0;
  background: none;
  text-decoration: none;

  cursor: pointer;
  outline: none;

  &-adversting {
    display: inline-block;
    padding: 5px 24px;
    border: 2px solid $primary-black;
    border-radius: 20px;
    overflow: hidden;
    position: relative;
    transition: color .3s ease-in-out, border .4s ease-in-out;

    &::before {
      background: linear-gradient(to right, $purple-medium 0%, $purple-medium 50%, $primary-black 50%, $primary-black 100%);
      content: "";
      position: absolute;
      top: 0;
      left: 0;

      width: 200%;
      height: 100%;
      z-index: -2;
      transition: all .3s ease-in-out;
    }

    &::after {
      content: '';
      width: 0;
      height: 0;
      border-style: solid;
      border-width: 0 0 40px 40px;
      border-color: transparent transparent $primary-black transparent;
      position: absolute;
      right: -5px;
      top: -5px;
      z-index: -1;
      transition: .3s ease-in-out;
    }

    &:hover::before {
      margin-left: -100%;
    }

    &:hover::after {
      right: 100%;
    }

    &:hover {
      color: $purple-medium;
    }
  }
}

// router-link, this is an a tag.

<router-link :to="linkTo" class="btn btn-adversting">
        Узнать больше
      </router-link>

The problem is this.