How to conditionally map two identical variables in nginx?

I would like to set the variable $sub conditionally dependent on $request_uri and $host.

map $request_uri $sub {
    default       front;
    ~^/api/.*     api;
    ~^/rest/.*    rest;
}

map $host $sub {
    hostnames;

    #default    $sub; # Tried, no success

    ping.*     ping;
    admin.*    admin;
}

server {
    listen 80;
    server_name _;

    root /usr/share/nginx/html;

    location ~ \.php$ {
        include snippets/fastcgi-php.conf;
        fastcgi_param HTTP_X_SUB $sub;
        fastcgi_pass unix:/run/php/php7.2-fpm.sock;
    }

    location / {
        try_files $uri $uri/ /index.php;;
    }
}

I thought nginx will not overwrite $sub in map $host $sub if the hostname is not in the list, but unfortunatly it just blanks out $sub.

So basically it should be like if $host is "ping" or "admin" set $sub accordingly. If hostname did not match, check for $request_uri.

Any idea how to archive that statement?

EDIT: Extended the config.