No redirection (rewrite) from one folder to another folder

The full .htaccess file

Following is my full htaccess file.

<IfModule mod_rewrite.c>
Options +FollowSymLinks
RewriteEngine On
RewriteBase /

#redirect index to homepage
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]

#prevent hotlinking
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http(s)?://(www.)?www.example.com [NC]
RewriteRule .(jpg|jpeg|png|gif)$ - [NC,F,L]

#redirect galleries to portfolio
RewriteRule ^galleries/(.*)$ /portfolio/$1 [R=301,NC,L]

#prevent access to wp-config
<files wp-config.php>
 order allow,deny
 deny from all
</files>
</IfModule>

The directive in question

The following one is obviously the one in question:

#redirect galleries to portfolio
RewriteRule ^galleries/(.*)$ /portfolio/$1 [R=301,NC,L]

Expected outcome

If I go to http://subdomain.example.com/galleries/headshots/ the server should redirect to http://subdomain.example.com/portfolio/headshots/

What happening instead

Nothing at all happens, I remain on http://subdomain.example.com/galleries/headshots/.

What I have tried so far without success

I added slashes at the end of both, the source and target – nothing changed:

#redirect galleries to portfolio
RewriteRule ^galleries/(.*)$/ /portfolio/$1/ [R=301,NC,L]

Also tried putting the directive on top, just below RewriteBase / – still no luck.

Notes

Note that this happens on a subdomain (see example url), although I don’t think that has an impact on this behaviour.

This is happening on a WordPress installation, but I have doubts it influences that. All the other directives seem to work fine (react on change).

Edit (from the comment reply to @Panama Jack): Both directories actually exist and are separate pages, but one is only setup as a script, if you like (other pages have to access its gallery).