Post preview page redirects to incorrect page

Hey hi,

below is my scenario which i was facing issue with.

I have hosted a blog in test.domain1.com/blog. And proxied the blog in to my root domain. Say like https://www.domain1.com/blog.

I have kept the home url as: test.domain1.com/blog and
the site url as: https://www.domain1.com/blog

Problem:

All the uses cases works; however, when after writing a post and tend to preview from the wpadmin panel it takes me https://www.domain1.com/blog/post page and so I end up with the following error.

Sorry, you are not allowed to preview drafts.

when modified the url with test.domain1.com/blog/post it works perfect.

Any solutions/thoughts on the above issue would be much appreciated.

Wordpress version: 5.1.1

Thank you!