WP Super Cache Dynamic Secret TAG visible on uncached first load

I am trying to utilise the dynamic caching features of WP Super Cache, but I’m having an issue and I can’t see how to fix it, currently, when my site loads for the first time, my secret key shows on front-end, when the user refreshes the page it will show the content

I found a similar issue on:
https://github.com/Automattic/wp-super-cache/issues/490

Example code:
https://github.com/Automattic/wp-super-cache/blob/master/plugins/dynamic-cache-test.php

Can you help me?