Почему flex дает ширину элемента hr равной 0?

* {
  /* Makes width and height include padding, border */
  box-sizing: border-box;
}

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

h1,
h2,
h3,
h4,
h5,
h6 {
  color: #2d3c49;
  text-transform: uppercase;
}

h1 {
  font-weight: 200;
  /* Browsers typically display h1 as 2em */
  font-size: 2.6em;
  margin-bottom: 0;
}


/* Adds a bit of space above subtitle (set h1 bottom margin to 0) */

header h4 {
  margin-top: 7px;
}


/* Top content */

header {
  display: flex;
  max-width: 1000px;
  margin: 0 auto;
  margin-top: 1em;
  /* Vertically centers logo  */
  align-items: center;
}


/* logo img */

.logo {
  height: 90px;
  width: auto;
  margin-right: auto;
}


/* Only subtitle isn't aligned all the way to the right; this fixes it. TODO: figure out why doesn't apply to h1 text */

.header-text {
  display: flex;
  justify-content: flex-end;
}

hr {
  background-color: #7d97ad;
  max-width: 1000px;
  /* margin-bottom: 1em; */
  border: none;
  height: 3px;
  margin: 0 0 1em 0;    /* NEW */
  align-self: stretch;  /* NEW */
}

.main-image {
  max-width: 1000px;
  height: auto;
}


/* Applies to content within <main> element (excludes header, footer) */

.container {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  max-width: 1000px;
  margin: 0 auto;
}


/* Applies to project section (including header text) */

.container-projects {
  display: flex;
  /* Parent container needs this for flex-item to take full width in row */
  flex-wrap: wrap;
  flex-direction: row;
  justify-content: space-between;
  margin: 2em 0;
}

.portfolio-header {
  /* Puts header in its own row without removing from container with row flex direction (setting parent container to wrap also required) */
  width: 100%;
  text-align: left;
  color: #7d97ad;
}


/* Div containing single project's title, image, text */


/* TODO: add declarations */

.project {
  width: 300px;
  height: auto;
}

.project p,
h3 {
  text-align: center;
}


/* Images cropped with 3:2 ratio, scaled resolution down to 600 x 400 */

.project-image {
  width: 300px;
  height: auto;
}

footer {
  text-align: center;
  margin-top: 1em;
  background-color: #ccc;
  color: white;
  padding: 2em;
  font-size: 1.1em;
}


/* Remove default 1em margin-top */

footer p {
  margin-top: 0;
}


/* Applies to Font Awesome social icons */

.fab {
  margin: 0 0.5em;
  color: white;
}


/* Changes social icon color to dark grey on hover */

.fab:hover {
  color: #2d3c49;
}


/* Media queries (breakpoints correspond to Bootstrap breakpoints). 1rem = 16px */


/* Small devices (landscape phones) */

@media screen and (max-width: 767px) {
  h1 {
    font-size: 2em;
  }
  h2 {
    font-size: 1.5em;
  }
  h3 {
    font-size: 1.3em;
  }
  h4 {
    font-size: 1.1em;
  }
  /* Doesn't seem to be doing anything TODO: find out why   */
  .portfolio-header {
    margin-bottom: 1em;
  }
  /* TODO: make slightly wider  */
  .container-projects {
    margin: 1.5em 0 0 0;
  }
  header {
    margin: 0 1em 0 0;
  }
  header,
  .container,
  footer {
    max-width: 100%;
  }
  /* Must specify max-width for img even though parent .container has the same declaration because max-width isn't inherited */
  .container img {
    max-width: 100%;
  }
  .project {
    /* Centers projects (aligned left otherwise) */
    margin: 0 auto;
  }
  /* Aligns portfolio header text flush left of portfolio rows */
  .portfolio-header {
    width: 300px;
    margin: 10px auto;
  }
  .logo {
    height: 50px;
    width: auto;
  }
}


/* Tablets */

@media screen and (max-width: 991px) {
  h1 {
    font-size: 1.7rem;
  }
}


/* Small laptops */

@media screen and (max-width: 1199px) {
  h1 {
    font-size: 2rem;
  }
}

<header>
  <img src = "https://image.ibb.co/jVeP4S/udacity_logo.png" alt = "Udacity logo" class = "logo">
  <!-- Header is set to display: flex, and it only works on direct children. Without the div, its default row alignment puts header text side by side, but inside a div, it works on the div as a block and since header text items are no longer direct children, it has no effect on them -->
  <div>
    <h1>Natalie Cardot</h1>
    <h4 class = "header-text">Front-End Web Developer</h4>
  </div>
</header>
<!-- TODO: put inside main? But makes it disappear -->

<main class = "container">
  <hr>
  <img src = "https://image.ibb.co/cTcuM7/using_laptop_large.jpg" alt = "Woman using laptop at home during the day" class = "main-image">
  <section class = "container-projects">
    <h2 class = "portfolio-header">Featured Work</h2>
    <div class = "project">
      <img class = "project-image" src = "https://image.ibb.co/hv4c8n/santorini_small.jpg" alt = "View from island of Santorini on a sunny day">
      <h3>Project No. 1</h3>
      <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</p>
    </div>
    <div class = "project">
      <img class = "project-image" src = "https://image.ibb.co/c9sKM7/coast_small.jpg" alt = "Distant view of a rugged island with a sailboat nearby">
      <h3>Project No. 2</h3>
      <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</p>
    </div>
    <div class = "project">
      <img class = "project-image" src = "https://image.ibb.co/eO9oES/mediterranean_small.jpg" alt = "Bird's eye view of a rocky beach with clear turquoise waters">
      <h3>Project No. 3</h3>
      <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</p>
    </div>
  </section>
  <!-- End of container-project -->
</main>
<footer>
  <p>Created by me</p>
  <!-- "fab" prefix new in Font Awesome version 5 (released 3/18) -->
  <a href = "https://github.com/nataliecardot" target = "_blank"><i class = "fab fa-github fa-lg"></i></a>
  <a href = "https://codepen.io/nataliecardot/"><i class = "fab fa-codepen fa-lg"></i></a>
  <a href = "https://twitter.com/nataliecardot/"><i class = "fab fa-twitter fa-lg"></i></a>
  <a href = "https://www.linkedin.com/in/natalie-cardot/" target = "_blank"><i class = "fab fa-linkedin-in fa-lg"></i></a>
</footer>