multi_query() has an error

I need some help finding my error on the enclosed code. When I run either of the two queries using the if ($conn->query($sql) === TRUE) { method each works correctly. But when I try to combine them with the if ($conn->multi_query($sql) === TRUE) { method. No records are uploaded. What am I doing wrong here.

    <?php
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "practice";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}
echo "Connection made...";

$payload_dump = $_POST['payload'];
echo $payload_dump;

$payload_array = json_decode($payload_dump,true);

if(is_array($payload_array)){
foreach($payload_array as $row){

//get the data_payload details

$device = $row['device'];
$type = $row['data_type'];
$zone = $row['zone'];
$sample = $row['sample'];
$count = $row['count'];
$time = $row['date_time'];
$epoch = $row['epoch_stamp'];

$sql = "INSERT INTO data(device, type, zone, sample, count, date_time, epoch_stamp) VALUES('$device', '$type', '$zone', '$sample', '$count', '$time', '$epoch');";
$sql . = "UPDATE data SET date_time = FROM_UNIXTIME(epoch_stamp);";
if ($conn->multi_query($sql) === TRUE) {
//if ($conn->query($sql) === TRUE) {
    echo "New record created successfully";
} else {
    echo "Error: " . $sql . "<br>" . $conn->error;
}
}
}
$conn->close();
?>

... and yes I realize this code is not secure but it's ok for my testing purposes.

Intrinsically the code below is the same until we get to the loop where we build up an array of queries to be executed and execute the multi_query() once at the end once we leave the loop. I have removed some of the comments and statements that echo out info at the start for brevity. I hope this looks ok and works....

<?php
    $servername = "localhost";
    $username = "root";
    $password = "";
    $dbname = "practice";

    $conn = new mysqli($servername, $username, $password, $dbname);
    if( $conn->connect_error ) die("Connection failed: " . $conn->connect_error);
    $payload_dump = $_POST['payload'];

    $payload_array = json_decode($payload_dump,true);

    if( is_array( $payload_array ) ){

        $queries=array();

        foreach( $payload_array as $row ){
            //get the data_payload details
            $device = $row['device'];
            $type = $row['data_type'];
            $zone = $row['zone'];
            $sample = $row['sample'];
            $count = $row['count'];
            $time = $row['date_time'];
            $epoch = $row['epoch_stamp'];

            /*note: we do not need to add the semi-colon here as it gets added later when we implode the array */
            $queries[]="INSERT INTO `data` ( `device`, `type`, `zone`, `sample`, `count`, `date_time`, `epoch_stamp` ) VALUES ('$device', '$type', '$zone', '$sample', '$count', '$time', '$epoch')";
        }
        /*
            Previously the below query was being execute on every iteration
            ~ because $epoch is now the last one encountered in the array, 
            the value that is updated in ALL records is as it would have been
            previously.
        */
        $queries[]="UPDATE `data` SET `date_time` = from_unixtime( $epoch );";

        $sql=implode( ';', $queries );
        if ( $conn->multi_query( $sql ) === TRUE ) {
            echo "New records created and updated successfully";
        } else {
            echo "Error: " . $sql . "<br>" . $conn->error;
        }
    }
    $conn->close();

?>