.htaccess headers being ignored by Apache

I have a website that uses the same core .htaccess details as many other websites; however this website does not properly load the .htaccess directives -- giving a basic HTTP header set of:

    HTTP/1.1 200 OK
    Date: Mon, 12 Nov 2018 09:34:28 GMT
    Server: Apache
    Keep-Alive: timeout=5, max=100
    Connection: Keep-Alive
    Transfer-Encoding: chunked
    Content-Type: text/html; charset=UTF-8

The website itself loads fine, but additonal headers in .htaccess are not being agknowledged / loaded.

So .htaccess is being read, right?

Yes -- The htaccess file contains HTTPS forced redirects and domain name redirects (from the .co.uk to .com address (both to the same website account))

These work.

Headers supplied by PHP are being loaded fine, too

The PHP headers on a test page are loading just fine:

<?php
header("Cache-Control: no-cache, must-revalidate");
header('Content-Type: text/html; charset=utf-8');
header("X-Clacks-Overhead: GNU Terry Pratchett");
header("Content-Language: en");
header("X-XSS-Protection: 1; mode=block");
header("X-Frame-Options: SAMEORIGIN");
header("X-Content-Type-Options: nosniff");
?>

But the same headers set in the .htaccess are not being agknowledged.

So it's an .htaccess syntax error!

Not that I can see; usually with a .htaccess error the site loads an HTTP-500 error message, however here the site loads in the browser without issue.

When there IS a deliberate syntax error the error-500 HTTP response comes back as expected.

Ok bozo, check your error logs!

Absolutely; I couldn't agree more. The Apache error logs are empty!

What have you tried to do to fix this?

• Confirmed httpd.conf allows reading of .htaccess
• Confirmed that mod_headers.c is loaded on the server
• Commented out and re-written various rules, to no effect
• Read lots (maybe 6-8) of posts on Stack Overflow and Server Fault - Stackoverflow posts don't appear to relate or their issues had distinct differences.
• Confirmed my .htaccess has the correct permissins (0644)
• Told my staff (He's a Graphic Designer).
• Cried myself to sleep.

Right then - Get your file out! Show me the magic!

Here:

Options +FollowSymLinks
Options -Indexes
RewriteEngine On
ErrorDocument 404 /index.php?msg=404
ErrorDocument 403 /index.php?msg=403

#Set asset items to cache for 1 week.
<FilesMatch "\.(gif|jpe?g|png|ico|css|js|swf|mp3)$">
     Header set Cache-Control "max-age=1972800, public, must-revalidate"
</FilesMatch>

RewriteCond %{HTTPS} !=on
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

## This does not appear to work (for either)
#Header always set Strict-Transport-Security "max-age=31536000;" env=HTTPS
Header always set Strict-Transport-Security "max-age=31536000; includeSubdomains;" "expr=%{HTTPS} == 'on'"
Header set Expect-CT enforce,max-age=2592000

RewriteCond %{HTTP_HOST} ^(www\.)?thewebsite\.co\.uk$ [NC]
RewriteRule ^/?(.*)$ https://www.thewebsite.com%{REQUEST_URI} [R=301,L]

###
##### Seems to workdown to roughly this point.
###

#force requests to begin with a slash.
RewriteCond  %{REQUEST_URI}  !^$
RewriteCond  %{REQUEST_URI}  !^/
RewriteRule  .*              -    [R=403,L]

RewriteCond %{HTTP_HOST} !^$
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^ https://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]

RewriteCond %{ENV:REDIRECT_STATUS} 200
RewriteRule .* - [L]

### This file does not exist on the directory at present. 
<Files .account-user.ini>
    order allow,deny
    deny from all
</Files>

###
#### None of these appear on assessment tools such as Security Headers 
#### Or redbot.
###
Header set Cache-Control no-cache,must-revalidate
Header set X-Clacks-Overhead "GNU Terry Pratchett"
Header set X-XSS-Protection 1;mode=block
Header set X-Content-Type-Options nosniff
Header always set X-Frame-Options SAMEORIGIN
Header set Expect-CT enforce,max-age=2592000
Header set Content-Language en
Header set Referrer-Policy origin-when-cross-origin

<LimitExcept GET POST HEAD>
    deny  from all
</LimitExcept>

And finally it would really help if you gave me a final summary of all of the above!

• Header setting commands in .htaccess do not appear to work.
• ALL parts of the file are used on other live sites elsewhere without issue.
• Headers can be set in PHP without issue
• No errors arise from these Headers in the .htaccess.
• Headers appear to fail silently.
• No Apache error logs are recorded.
• The .htaccess is being read by Apache because other commands (such as mod_Rewrites) are being actioned

So it looks like my .htaccess can't set headers. How can I fix this?

---

UPDATE:

From research by other parties (the hosting providers) it seems that somehow the .htaccess works and loads all the correct headers for non PHP pages.

For even plain PHP pages; the headers are blank.

from .htaccess headers being ignored by Apache