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wordpress sub-category ( lowercase letters + non-latin characters ) = 404
If the sub-category's slug is a combination of lowercase letters and Korean, page 404 appears.

1 : NGINX settings
location / {
    try_files $uri $uri/ /index.php?$args;
}

2 : Wordpress - Permerlink settings

Custom: /%category%/%postname%/
Category Base:.

3 : Wordpress - Yoast

Taxonomies > Category URLs ( Remove )

4 : Tested content
4-1 : In case of parent category ✅

mywebsite.com/한글 = 200
mywebsite.com/a한글 = 200
mywebsite.com/한글B = 200

4-2 : In case of post ✅

mywebsite.com/한글/post글 = 200 ( lowercase + non-latin )

4-3 : In case of child category ⚠️

mywebsite.com/한글/한글  = 200
mywebsite.com/한글/a한글 = 404 ❌
mywebsite.com/한글/한글b = 404 ❌
mywebsite.com/한글/C한글 = 200
mywebsite.com/한글/한글D = 200
mywebsite.com%2F%ED%95%9C%EA%B8%80%2Fa%ED%95%9C%EA%B8%80 = 200 ⭕️ ( /한글/a한글 : url encoded )
mywebsite.com/한글/A한글 = 200 ⭕️ ( /한글/a한글 : uppercase )

5 : Reference question
https://wordpress.stackexchange.com/questions/277401/arabic-characters-in-url-returns-404-error
I think it might be related to @user139986 's answer.
I don't know how to handle this problem .. 

Looking at @user139986's answer (linked in your question): In the .htaccess file, the NC indicates no case, ie, the request should be considered in a case-insensitive manner. This means that, say, ABC = abc = AbC. Does this question and its answer(s) help you out?

@PatJ Thanks for explaining. I guess I misunderstood that it has something to do with user139986's answer.  The lowercase letters are the problem, so it doesn't seem to be related.

⚠️ Temporary workaround
https://stackoverflow.com/a/59109698/15497563
sudo apt-get install nginx-extras

nginx.conf
https://stackoverflow.com/a/11170826/15497563
http {
    # Include the perl module
    perl_modules perl/lib;

    # Define this function
    perl_set $uri_uppercase 'sub {
        my $r = shift;
        my $uri = $r->uri;
        $uri = uc($uri);
        return $uri;
    }';
}

site-enabled
https://stackoverflow.com/a/35369570/15497563
server {
    location ~ ^/(부모/자식a|부모/자식b|부모/자식c)(.*) {
        rewrite ^(.*)$ $scheme://$host$uri_uppercase;
    }
}

Every time you create a sub-category(lowercase letters + non-latin characters), you need to edit it...
