Why does a 404 error occur when the Russian letter “X” appears in the URL?

Question

URL Handling

I need to pass a region as a GET parameter, but when I pass the following names of regions as it:
Sakhalin region, Sakha, Astrakhan region, Arkhangelsk region, I get a 404 error .
Through a simple analysis, I found out that such a reaction is observed when the letter "X" is present in the URL . Why is this happening?

Here is a CNC implementation and an example link:
CNC:

AddDefaultCharset UTF-8
Options +FollowSymLinks
RewriteBase /
RewriteEngine On
RewriteRule ^region/(.+?)$ /pages/region.php?reg=$1 [L]

Link example: mysite.ru/region/Arkhangelsk%20 region

Answer 1

/pages/region.php is missing
or it is there but it returns this title itself