Flash message in PHP clearing $_SESSION key before it is supposed to.

I'm having a problem with what is called I think a Flash Message.
It should clear a key from the $_SESSION array after outputting it, but it clears it before. I know because if I comment the unset() it works (but of course key isn't cleared and persist on refresh) In short this would be the code:

if (isset($_SESSION['error'])) {
   echo('<p style="color:red; margin:0">'. $_SESSION['error'] .'</p>');
   unset($_SESSION['error']); 
}

As it is the echo statement doesn't work. If comment the unset the echo works but the $_SESSION key isn't cleared and i need to clear it after output. Extremely weird, i coded the same in other files and it works flawlessly. Here the whole code for the riddle lovers.

<?php 
session_start();
if(isset($_POST['username']) && isset($_POST['password'])){
   unset($_SESSION['username']);
   if($_POST['password'] =='umsi' && $_POST['username'] == 'chuck') {
      $_SESSION['username'] = $_POST['username'];
      $_SESSION['success'] = 'Logged in successfuly';
      $_SESSION['password'] = $_POST['password'];
      header('Location: app.php');
      return;
   }
   else {
      $_SESSION['error'] = 'Login incorrect';
      header('Location: log.php');
   }
}
?>
<!DOCTYPE html>
<html lang="es">

<head>
   <meta charset="UTF-8">
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <meta http-equiv="X-UA-Compatible" content="ie=edge">
   <link rel="stylesheet" href="main.css">
   <title>Log In</title>
</head>

<body>
   <div class="log">
      <?php
      if (isset($_SESSION['error'])) {
         echo('<p style="color:red; margin:0">'. $_SESSION['error'] .'</p>');
         unset($_SESSION['error']);
      }
      ?>
      <h1>Log in</h1>
      <form method="post">
         <label for="username">Username</label>
         <input type="text" name="username">
         <label for="password">Password</label>
         <input type="password" name="password">
         <button type="submit">Submit</button>
      </form>
   </div>
</body>

</html>

Solution

Thanks to Lawrence Cherone I could realize i missed the return at the end of the else satement.

 else {
      $_SESSION['error'] = 'Login incorrect';
      exit(header('Location: log.php'));
   }

now it works!!