Почему углы обрезаются на ходу?

$('.menu-btn').on('click', function(e) {
  e.preventDefault();
  $(this).toggleClass('menu-btn_active');
  $('.menu-nav').toggleClass('menu-nav_active');
});

body {
  font-family: sans-serif;
  margin: 0;
}

.section {
  height: 100vh;
  background-color: #7b1fa2;
  display: flex;
  justify-content: center;
  align-items: center;
}

.menu-btn {
  display: block;
  width: 50px;
  height: 50px;
  background-color: #fff;
  border-radius: 50%;
  position: relative;
}

.menu-btn span,
.menu-btn span::before,
.menu-btn span::after {
  position: absolute;
  top: 50%;
  margin-top: -1px;
  left: 50%;
  margin-left: -10px;
  width: 20px;
  height: 2px;
  background-color: #222;
}

.menu-btn span::before,
.menu-btn span::after {
  content: '';
  display: block;
  transition: .2s;
}

.menu-btn span::before {
  transform: translateY(-5px);
}

.menu-btn span::after {
  transform: translateY(5px);
}

.menu-btn_active span {
  height: 0;
}

.menu-btn_active span::before {
  transform: rotate(-45deg);
}

.menu-btn_active span::after {
  transform: rotate(45deg);
}

.menu-block {
  display: flex;
  justify-content: center;
  align-items: center;
  overflow: hidden;
  border-top-right-radius: 50px;
  border-bottom-right-radius: 50px;
}

.menu-nav {
  background-color: #fff;
  text-decoration: none;
  height: 50px;
  text-decoration: none;
  border-top-left-radius: 50px;
  border-bottom-left-radius: 50px;
  padding-left: 20px;
  padding-right: 45px;
  margin-right: -45px;
  transition: .5s;
  transform: scaleX(0) translateX(0%);
  transform-origin: right center;
  opacity: 0;
  line-height: 50px;
}

.menu-nav__link {
  display: inline-block;
  text-decoration: none;
  color: #222;
  margin-right: 20px;
}

.menu-nav_active {
  transform: scaleX(1) translateX(0%);
  opacity: 1;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="section">
  <div class="menu-block">
    <div class="menu-nav">
      <a class="menu-nav__link" href="#">Home</a>
      <a class="menu-nav__link" href="#">Portfolio</a>
      <a class="menu-nav__link" href="#">About</a>
      <a class="menu-nav__link" href="#">Contacts</a>
    </div>
    <a href="#" class="menu-btn">
      <span></span>
    </a>
  </div>
</div>