Why ajax request sometimes returns html page?

Question

JavaScript

In an application, the backend is separated from the frontend. The entry point for the front is index.html and the backend is api.php. The front communicates with the backend through ajax requests, but some requests apparently go to index.html, resulting in an html page in the response. Requests are regulated by .htaccess, maybe I didn't configure it correctly:

AddDefaultCharset utf-8
Options -Indexes

RewriteEngine On
RewriteBase /

# if it is ajax request
RewriteCond %{HTTP:X-Requested-With} XMLHttpRequest
RewriteRule . api.php [L]

# if a directory or a file exists, use it directly
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . index.html

Some requests are treated by adding a slash to the end of the address: site.ru/users/1 -> site.ru/users/1/
Now it is the problem with POST requests

Answer 1

Check the headers being sent, as you look only at them, the point is most likely in this