CSSのflexboxでjustify-contentのspace-betweenが効かないサンプル

justify-content: space-between

最後のdivにmargin-rightが指定されている

justify-content: space-around

::before, ::after content: ""

<section>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
</section>

<section>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div class="s1"></div>
</section>

<section class="s2">
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
</section>

<section class="s3">
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
</section>

section {
  display: flex;
  justify-content: space-between;
  width: 300px;
  margin-bottom: 30px;
  background: #fff333;
}

section > div {
  width: 40px;
  height: 40px;
  background: #f00;
}

.s1 {
  margin-right: 10px;
}

.s2 {
  justify-content: space-around;
}

.s3::before,
.s3::after {
  content: "";
}