$_SESSION['message] not working when using $_GET

Question

I've followed a CRUD tutorial (https://www.youtube.com/watch?v=3xRMUDC74Cw&ab_channel=CleverTechie) and have everything working, but the delete message, when I delete a record. I thought maybe the button class danger was not working (outdated or w/e) for some reason and tried to change it to something else, but it still doesn't work.

Now I believe it has to do something with the fact I'm using $_GET method for deleting the records, but I'm not sure anymore. The success message and updated message are both working perfectly, but the delete message isn't.

index.php

<?php require_once 'process.php'; ?>

<?php if(isset($_SESSION['message'])): ?>

<div class="alert alert-<?=$_SESSION['msg_type']?>">

    <?php 
        echo $_SESSION['message'];
        unset($_SESSION['message']);
    ?>

</div>
<?php endif ?>

<div class="container">

    <?php
        $mysqli = new mysqli('localhost', 'root', '', 'dbase') or die(mysqli_error($mysqli));
        $result = $mysqli->query("SELECT * FROM data") or die($mysqli_error);
        //pre_r($result);
    ?>

    <div class="row justify-content-center">
        <table class="table">
            <thead>
                <tr>
                    <th>Name</th>
                    <th>Location</th>
                    <th colspan="2">Action</th>
                </tr>
            </thead>

            <?php while ($row = $result->fetch_assoc()): ?>

                <tr>
                    <td><?php echo $row['name']; ?></td>
                    <td><?php echo $row['location']; ?></td>
                    <td>
                        <a href="index.php?edit=<?php echo $row['id']; ?>"
                           class="btn btn-info">Edit
                        </a>
                        <a href="index.php?delete=<?php echo $row['id']; ?>"
                           class="btn btn-danger">Delete
                        </a>
                    </td>
                </tr>

            <?php endwhile; ?>
        </table>
    </div>

    <?php
        //pre_r($result->fetch_assoc());
        //pre_r($result->fetch_assoc());

        function pre_r($array) {
            echo '<pre>';
            print_r($array);
            echo '</pre>';
        }
    ?> 

    <div class="row justify-content-center">
        <form action="process.php" method="POST">
            <input type="hidden" name="id" value="<?php echo $id; ?>">
            <div class="form-group">
                <label>Name</label>
                <input type="text" name="name" class="form-control" 
                       value="<?php echo $name; ?>" placeholder="Enter your name">
            </div>
            <div class="form-group">
                <label>Location</label>
                <input type="text" name="location" class="form-control" 
                       value="<?php echo $location; ?>" placeholder="Enter your location">
            </div>
            <div class="form-group">
                <?php 
                    if ($update == true):
                ?>
                    <button type="submit" class="btn btn-info" name="update">Update</button>
                <?php else: ?>
                    <button type="submit" class="btn btn-primary" name="save">Save</button>
                <?php endif; ?>
            </div>
        </form>
    </div>
</div>

process.php

<?php

session_start();

$mysqli = new mysqli('localhost', 'root', '', 'pdlenart') or die(mysqli_error($mysqli));

$update = false;
$id = 0;
$name = "";
$location = ""; 

if (isset($_POST['save'])) {
    $name = $_POST['name'];
    $location = $_POST['location'];

    $mysqli->query("INSERT INTO data (name, location) VALUES('$name', '$location')") or die($mysqli_error);
        
    $_SESSION['message'] = "Record has been saved!";
    $_SESSION['msg_type'] = "success";

    header("location: index.php");
}

if (isset($_GET['delete'])) {
    $id = $_GET['delete'];

    $mysqli->query("DELETE FROM data WHERE id=$id") or die($mysqli_error);

    $_SESSION['message'] = "Record has been deleted!";
    $_SESSION['msg_type'] = "danger";

    header("location: index.php");

}

if (isset($_GET['edit'])) {
    $id = $_GET['edit'];
    $update = true;
    $result = $mysqli->query("SELECT * FROM data WHERE id=$id") or die($mysqli_error);

    if (count($result)==1) {
        $row = $result->fetch_array();
        $name = $row['name'];
        $location = $row['location'];
    }
}

if (isset($_POST['update'])) {
    $id = $_POST['id'];
    $name = $_POST['name'];
    $location = $_POST['location'];

    $mysqli->query("UPDATE data SET name='$name', location='location' WHERE id=$id") or die($mysqli_error);

    $_SESSION['message'] = "Record has been updated!";
    $_SESSION['msg_type'] = "warning";

    header("location: index.php");
}

EDIT: I've made it work by removing header(location: index.php); in the if (isset($_GET['delete'])) {} section. Not sure what happens or why.

Also I'll keep in note the comments about SQL injection, but this is about why certain functionality isn't working or is as it just happened by removing header(Location..).