Wordpress Wordpress cached images issue

I have a problem with images sometimes rendering far larger than they should. I've checked the code and when an image loads correctly I see this:

HTML:

<img class="preload-me lazy-load is-loaded" src="https://site.co.uk/wp-content/uploads/2017/07/x61wKIUHA-yL._SL1500_-60x60.jpg.pagespeed.ic.QQA7eGmZLu.webp" alt="" style="will-change: auto;" srcset="https://site.co.uk/wp-content/uploads/2017/07/61wKIUHA-yL._SL1500_-60x60.jpg 60w, https://site.co.uk/wp-content/uploads/2017/07/61wKIUHA-yL._SL1500_-120x120.jpg 120w" width="60" height="60">

After a few page refreshes an image may be rendered far too large and the code shows the following:

Code:

<img class="preload-me lazy-load is-loaded" src="data:image/webp;base64,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" alt="" data-pagespeed-url-hash="1322903256" onload="pagespeed.CriticalImages.checkImageForCriticality(this);" style="will-change: auto;" srcset="https://site.co.uk/wp-content/uploads/2017/07/61wKIUHA-yL._SL1500_-60x60.jpg 60w, https://site.co.uk/wp-content/uploads/2017/07/61wKIUHA-yL._SL1500_-120x120.jpg 120w">

Any ideas what could be causing this issue, maybe pagespeed?

custom_config.inc:

Code:

NGXDYNAMIC_NGXPAGESPEED='y'
NGINX_PAGESPEED='y'
PHP_PGO='y'
PHP_PGO_CENTOSSIX='y'
LETSENCRYPT_DETECT='y'
VHOSTCTRL_CLOUDFLAREINC='y'
NGXDYNAMIC_BROTLI='y'
NGINX_LIBBROTLI='y'
NGINX_HPACK='y'
NGINX_DYNAMICTLS='y'
CLOUDFLARE_ZLIBPHP='y'
CLOUDFLARE_PATCHSSL='y'
OPENSSLECDSA_PATCH='y'
OPENSSLECDHX_PATCH='y'
MARCH_TARGETNATIVE='y'
ENABLE_MARIADBTENTWOUPGRADE='y'
ENABLE_MARIADBTENTHREEUPGRADE='y'

pagespeed.conf: https://gist.githubusercontent.com/...60e39e8c1b0290ed40c7144503320e/pagespeed.conf

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Do you know why some of these small thumbnail images sometimes get rendered massively over size when they have been converted to these data uris?

For example this should be a tiny thumbnail next to the latest blog post listed in the footer: Capture

This is happening on multiple different sites on the same server.

 

Can I just disable that feature of ngx_pagespeed?

 

Sorted, thank you.