After upgrading our PROD environment to 15.4, we found that the New UX is different between PROD and DEV even though they are at the same build.
Typically this is caused by the browser using older cached files to display the pages. The cached versions of those pages may not have the updated features/code.
The issue is resolved by clearing the browser cache, and then logging back into CA PPM. We always recommend clearing the browser cache after an upgrade completed, or a patch is applied to your PPM environment. This will help prevent these types of issues from occurring.