Well my friends... After 4.5 years, the issue still persists!
I'm using --wprocket along with Perfmatters with changing login URL to something else, so it's not the normal wp-admin login URL. After i hit LOGIN button, the 405 Not Allowed appeared.
It's not the same issue though. You're changing the login url which is non-standard WordPress behaviour.
There's 3 ways round it: 1. don't change the login url, 2. use the custom Rocket NGINX config as you said (takes 2 minutes to set up), or 3. don't use WP Rocket (Nginx FastCGI does the same thing natively, add a couple of extra plugins if you want things like minifiy or database optimization etc).