Thanks for your feedback Yui.
Cache clearing issues are side-effect of linux security.
When nginx and php user are different AND php user are not part added to nginx group, you may notice this issue.
Reason - cached pages are created by Nginx so they are owned by nginx-user.
If php user is different, it relies on group or other users write permission for cached content.
Ideal workaround would be from Nginx module. The nginx module which creates cache should also provide an API/function call to purge entire cache.
Till then, on some systems, some kind of user/group permission fixing (one-time) need to be done.