Strapi v.3.6.5 - draft content not found

Hi - I'm from a backend development background - so this issue is possibly due to lacking javascript knowledge.

Having installed the preview plugin - this works for cloning, and getting published content - via the preview API using a url like this:
/preview-content/custom-pages/2

However - for draft content - we get a 404 and the message: "Preview not found for given content type and Id".

Trying to debug the plugin - I can see that that line 131 in preview.js is returning null:

However I am unable to identify why this is.

Any help much appreciated!