Apache mod_rewrite - unwanted redirect instead of rewrite

I have an issue with mod_rewrite and I can't seem to solve it. I stripped the example down to the bare bones and I don't understand why a specific rule forces my browser to redirect instead of rewrite:

RewriteEngine on
#if request is for a physical-file OR for one of the language paths - skip (return as-is)
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} -f [OR]
RewriteCond %{REQUEST_URI} ^/de [OR]
RewriteCond %{REQUEST_URI} ^/en-US
RewriteRule ^ - [L]
#otherwise: rewrite to en-US folder
RewriteRule ^(.*)$ /en-US/$1 [NC,L,QSA]

I read the documentation very carefully and it seems like this should actually rewrite every call, so https://example.com/fuBar.html should actually retrieve the file /en-US/fuBar.html from my server - the users browser shouldn't know about it.

What's really happening is that for some reason the browser is redirected to https://example.com/en-US/fuBar.html. While this does display the correct content, it's just not what I want or what I thought this RewriteRule should do. What am I doing wrong?

*add - the .htaccess of the subfolders de and en-US:

RewriteEngine On
# If an existing asset or directory is requested go to it as it is
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} -f [OR]
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} -d
RewriteRule ^ - [L]
# If the requested resource doesn't exist, use index.html
RewriteRule ^ /index.html

Solution

There's nothing in the code you've posted that would trigger an external "redirect".

Make sure you have cleared your browser (and any intermediary) cache(s) to ensure you are not seeing an earlier/erroneous 301 (permanent) redirect. (301 redirects are cached persistently by the browser.)

Check the "network traffic" in the browser's developer tools to see the precise nature of this redirect to see what it redirects from/to, and well as the 3xx HTTP status code of the redirect (if indeed this is an external redirect).

It would seem the front-end (JavaScript/Angular) is manipulating the URL in the address bar (there is no redirect). From comments:

Actually there was no redirect happening at all! Rather since I set <base href="/en-US"> somehow my frontend (Angular) seems to have outsmarted me, manipulating the address without me realizing it. Turns out I don't even need to change the base href, I just need the rewrites.