Visitors accessing the site with a preview link are served cached content from Varnish.
Reproduction requires an environment with Varnish configured. This is reproducible in Acquia Cloud.
1) Generate a preview link for a workspace
2) Access the preview link in an incognito or non-logged in browser
You will be served whatever version of the page is cached.
Consider having the preview controller set other cookie values / a session cookie to bypass cache.
Recommend VCL configuration that can act on the wse_preview cookie parameter.
Active
2.0
Workspace Preview
Had the same issue with Platform.sh
To fix this, I had to add the cookie wse_preview to the .platform/routes.yaml settings file as per https://docs.platform.sh/define-routes/cache.html
I'd imagine the solution will be similar for Acquia as well.
This should be mentioned in the readme somewhere for the future folks.