nginx location directive does not return http status

Note: I have read through the several Q/A's on SO, ServerFault, etc... but none of the answer seem to be working.

Running an AWS ec2 Ubuntu (v20.04) web server with a basic nginx (v1.18) config. However, the location directive for /a-custom-path returning an http status code of 200 is never processed and/or is processed and ignored.

Here's the config:

server {
    listen 80 default_server;
    listen [::]:80 default_server;
    server_name _;

    location /a-custom-path {
      access_log off;
      add_header Content-Type text/plain;
      return 200 'OK';
    }

    access_log /var/log/nginx/unknown.log;
    return 412;
}

The location /a-custom-path is never returning a 200 status; it always falls through to the 412 response. I've tried various location directives such as:

location /a-custom-path
location /a-custom-path/
location = /a-custom-path
location = /a-custom-path/

However, they all have the same behavior of falling through to the 412 response.

Question
How can I create a custom location path that returns a 200 response and all other requests return a 412 response?

Edit - Revised & Working Config

For anyone else finding this question, here is the final working solution based on the accepted answer:

server {
    listen 80 default_server;
    listen [::]:80 default_server;
    server_name _;

    # everything except '/a-custom-path' gets a 412 response
    location / {
      return 412;
    }

    location /a-custom-path {
      return 200 'OK';
      access_log off;
      add_header Content-Type text/plain;
    }

    access_log /var/log/nginx/unknown.log;
}