Content type changes when cached, html downloads as file

I have WP Fastest Cache installed on several sites usually without much of a problem. This specific site is having trouble’s that even other sites on the same CPanel server don’t have.

So on the first hit the page is loaded correctly.
Headers appears normal with correct content-type:

HTTP/1.1 200 OK
Date: Mon, 26 Aug 2019 21:08:24 GMT
Server: Apache
Link: <https://************/wp-json/>; rel="https://api.w.org/", <https://www.*****.com/?p=13>; rel=shortlink
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive
Content-Type: text/html; charset=UTF-8
Content-Length: 26775

On the second and any subsequent requests when it pulls from cache, the page downloads as an html file. This happens regardless of browser as long as you are NOT logged into the wordpress admin. Chrome debugger lists the content-type as ‘document’ but more in depth packet captures show that the server is returning ‘application/x-httpd-php5’ along with the cache related headers specified by the plugins .htaccess entries.

HTTP/1.1 200 OK
Date: Mon, 26 Aug 2019 21:05:52 GMT
Server: Apache
Vary: Cookie,User-Agent,Accept-Encoding
Last-Modified: Mon, 26 Aug 2019 21:03:35 GMT
Accept-Ranges: bytes
Cache-Control: max-age=0, no-cache, no-store, must-revalidate
Pragma: no-cache
Expires: Mon, 29 Oct 1923 20:30:00 GMT
Content-Length: 26840
Keep-Alive: timeout=5, max=98
Connection: Keep-Alive
Content-Type: application/x-httpd-php5

So far I don’t see any conflicts but just in case here is a list of plugins.

+-----------------------------------+----------+-----------+---------+
| name                              | status   | update    | version |
+-----------------------------------+----------+-----------+---------+
| admin-custom-login                | active   | none      | 2.9.5   |
| advanced-custom-fields-pro        | active   | none      | 6.3.3.2 |
| akismet                           | inactive | none      | 4.1.2   |
| broken-link-checker               | active   | none      | 1.11.8  |
| contact-form-7                    | active   | none      | 5.1.4   |
| google-analytics-dashboard-for-wp | active   | none      | 5.3.9   |
| google-sitemap-generator          | inactive | none      | 4.1.0   |
| iwp-client                        | active   | none      | 1.9.4.1 |
| query-strings-remover             | inactive | none      | 1.1     |
| really-simple-captcha             | active   | none      | 2.0.2   |
| scripts-to-footerphp              | inactive | none      | 0.6.3   |
| simple-custom-post-order          | active   | none      | 2.4.4   |
| sucuri-scanner                    | inactive | none      | 1.8.21  |
| types                             | active   | available | 2.2.2   |
| wordfence                         | active   | none      | 7.4.0   |
| wp-fastest-cache                  | active   | none      | 0.8.9.7 |
| wp-script-optimizer               | inactive | none      | 0.4.5   |
| wordpress-seo                     | active   | none      | 11.9    |
| mu-iwp-client                     | must-use | none      |         |
+-----------------------------------+----------+-----------+---------+