JSFiddle - React, Tailwind, and code Playground

HTML

<div class="block">

  <div class="item">1</div>
  <div class="item">2</div>
  <div class="item item-b">3</div>
  <div class="item">4</div>
  <div class="item">5</div>
  <div class="item">6</div>

</div>


<div class="block">

  <div class="item">1</div>
  <div class="item">2</div>
  <div class="item">3</div>
  <div class="item">4</div>
  <div class="item item-b">5</div>
  <div class="item">6</div>
  <div class="item">7</div>

</div>


<div class="block">

  <div class="item">1</div>
  <div class="item">2</div>
  <div class="item">3</div>
  <div class="item">4</div>
  <div class="item">5</div>
  <div class="item">6</div>
  <div class="item item-b">7</div>
  <div class="item">8</div>
  <div class="item">9</div>
  <div class="item">10</div>

</div>


<div class="block">

  <div class="item">1 <strong>а вот тут начнутся проблемы с шириной</strong> </div>
  <div class="item">2</div>
  <div class="item item-b">3</div>
  <div class="item">4</div>
  <div class="item">5</div>
  <div class="item">6</div>
  <div class="item">7</div>

</div>

CSS

@import url('https://fonts.googleapis.com/css2?family=Neucha&display=swap');


* {
  box-sizing: inherit;
}

html {
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: 'Neucha', cursive;
}

.block {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
  margin-bottom: 40px;
}

.item {
  flex-grow: 1;
  padding: 10px;
  background: #eee;
  text-align: center;
}
.item-b {
  width: 100%;
  background: #ddd;
}
.item-b ~ .item {
  width: 100%;
}