I run through exact same issue. Later I come to know that custom post type I created was not public and hence it does not have ‘Preview Changes’ button visible.
Prior version of WordPress was displaying that button but it seems they have fixed it now.
To fix it, make sure you set ‘public‘ to ‘true‘ in array of arguments you are passing to ‘register_post_type function‘.
Hope it will help!
Neerav