Preview not available on front end

**Problem:**
We have been using the strapi-plugin-preview-content plug for a long time and never had any issues, but something has recently changed and we are no longer able to preview content. [The button](https://www.screencast.com/t/NFjq9nLJU) now just takes us to the same page a user without permissions would see and we have no idea why this is happening. It seams to be doing a redirect now from the preview URL (/api/preview?contentType=article&secret=preview&slug=test-article).

Any help or pointers would be much appreciated thanks!

**Video Overview:**
https://www.screencast.com/t/4uGmmTCIiez

**User permissions:**
https://www.screencast.com/t/Gaec4AGiO

- React app
- Strapi 3.6.8