Allow REST API over HTTP, the rest of the site forced to HTTPS

Question

I have a website with some custom api endpoints. I recently installed SSL and configured the website to redirect http visitors to https. I usually do this with the Really Simple SSL plugin.

The https version of the api works as intended. However, for historic reasons, i still have so support for some time an old application which uses the http api endpoint. Calling the http api result in a 404 error.

So i tried to disable the .htaccess automatic rewrite of the plugin and write it by hand, this is what wrote:

<IfModule mod_rewrite.c>
   RewriteCond %{HTTPS} !=on
   RewriteCond %{REQUEST_URI} !^/wp-json.*
   RewriteRule ^(.*) https://%{SERVER_NAME}/$1 [R,L]
</IfModule>


# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

When I do this the website pages get redirected to https correctly. The HTTPS API works, but the HTTP API version gets oddly redirected to the homepage.

Any help on this?