Completely bizarre behaviour on MYSQLi update - php

Question

A warning up front, this project is part of a reporting system for workers looking after people with some profound disabilities and there is some unsavoury language that I don't use in jest and wouldn't in such a forum unless it was completely pertinent. No offense is meant.
I don't know if I even expect an answer to this because it is so completely strange, but here goes... I have a pretty simple form editing a text field in a mysql database. The update code couldn't be simpler;
(after connecting to the database)
$stmt = $mysqli->prepare("UPDATE rep_evening SET rppe_notes = ? WHERE rppe_mrep = ?");
$stmt->bind_param('si', $_POST['enotes'], $_POST['repid']);
$stmt->execute();
$stmt->close();
header("Location: ../reports.php");
The code is probably pointless because it all works perfectly well UNTIL the words 'suck dicks' is somewhere in the text. Then it throws a 403 error which is seemingly triggered on the header return because the update works. I thought perhaps somebody had put a profanity filter in but if they did it's pretty useless because it doesn't matter what else is in there, and believe me there's some pretty unsavoury stuff, just those two words, with the plural on the latter word - the singular doesn't fire the error. The 2 words can be separated by up to 3 words and trigger the 403, but no more than 3.
There's nothing in error logs, no clues anywhere but it can be replicated any number of times.
MYSQL V5.6.30, PHP 5.4 if that's of interest.
Edit...
Here's the full processing page, there's 3 sections to the report and an approval tag for it to be viewed beyond admins;
<?php
include ('../includes/dbconn.php');
if (mysqli_connect_errno()) {
printf("Connect failed: %s\n", mysqli_connect_error());
exit();
}
$stmt = $mysqli->prepare("UPDATE rep_day SET rppm_notes = ? WHERE rppm_mrep = ?");
$stmt->bind_param('si', $_POST['mnotes'], $_POST['repid']);
$stmt->execute();
$stmt->close();
$stmt = $mysqli->prepare("UPDATE rep_evening SET rppe_notes = ? WHERE rppe_mrep = ?");
$stmt->bind_param('si', $_POST['enotes'], $_POST['repid']);
$stmt->execute();
$stmt->close();
$stmt = $mysqli->prepare("UPDATE rep_overnight SET rppo_notes = ? WHERE rppo_mrep = ?");
$stmt->bind_param('si', $_POST['onotes'], $_POST['repid']);
$stmt->execute();
$stmt->close();
$stmt = $mysqli->prepare("UPDATE mreport SET mrep_approved = ? WHERE mrep_id = ?");
$stmt->bind_param('si', $_POST['approved'], $_POST['repid']);
$stmt->execute();
$stmt->close();
header("Location: ../reports.php");
?>