PHP redirecting before finish MySQL insert - php

Question

I have this code on "insert.php":
if ($stmt = $GLOBALS['mysqli']->prepare("INSERT INTO table1(iduser, title, msg, anonim, ip, iduniversity) VALUES (?,?,?,?,?,?)")) {
$stmt->bind_param("issisi", $_SESSION['iduser'], $_POST['title'], $_POST['msg'], $anonim, getIP(), $_SESSION['iduniversity']);
if ($stmt->execute()) {
$idmsg = $GLOBALS['mysqli']->insert_id;
$i = 0;
$stmt2 = $GLOBALS['mysqli']->prepare("INSERT INTO tag(idmsg, tag) VALUES(?,?)");
foreach ($tags as $tag) {
if ($tag != '') {
$stmt2->bind_param("is", $idmsg, $tag);
if ($stmt2->execute()) {
$i++;
}
}
}
$stmt2->close();
$stmt->close();
mysqli_close($GLOBALS['mysqli']);
sendFile($idmsg);
header("Location: /public/watch_msg.php?idmsg=" . $idmsg);
exit();
} else {
exit("Ops! Ocorreu algum erro. COD1370");
}
} else {
exit("Ops! Ocorreu algum erro. COD1371");
}
So, everything is working fine, except that sometimes when it redirects to "watch_msg.php" the message seems not to be on the database yet. When this happens, as soon as I refresh the page, everything is there!
First thing I thought is that there could be a race-condition somewhere, but I read in another question that PHP is sequential, and as I close both statements and connection before the redirect (so that used tables should be unlocked), why i'm getting this result somethimes? What i'm doing wrong?
Also, no functions outputs anything, but "sendFile" saves an image if the user sends one, so headers should be fine (it also gives me the error when I comment the function).
Code on watch_msg:
$msg = NULL;
$tags = NULL;
$coments = NULL;
$data_high = date("Y-m-d H:i:s");
$iduser;
if ($loggedin) { //If logged in
$idmsg = filter_input(INPUT_GET, 'idmsg', FILTER_SANITIZE_STRING);
$iduser = $_SESSION['iduser'];
$query = "SELECT * FROM table1 WHERE iduser = ? AND idmsg = ? AND datemsg < ?";
$stmt = $GLOBALS['mysqli']->prepare($query);
$stmt->bind_param("iis", $iduser, $idmsg, $data_high);
if ($stmt->execute()) {
$msg = mysqli_fetch_assoc($stmt->get_result());
if ($msg === NULL) {
exit('This message doesn\'t exists');
}
...
} else {
echo "Error.";
}
}