Mod_Rewrite unexpected behavior L flag

You are missing an important fact about the L flag:

It is therefore important, if you are using RewriteRule directives in one of these [.htaccess, <Directory>] contexts, that you take explicit steps to avoid rules looping, and not count solely on the [L] flag to terminate execution of a series of rules

From: L|last (Flag); bold by me

That means, only by using the L does not have your desired effect to prevent the interal redirect. The INTERNAL REDIRECT happens here because it must happen, you have specified it with your .htaccess configuration. The L flag is not the right flag to prevent the INTERNAL REDIRECT.

Let’s look closer at your question and what actually happens:

I can’t understand it because I’ve placed the [L] flag just to ensure that the mod_rewrite stops.

Is just that you have the wrong undestanding of the L flag. It will only stop for the current rewriting, meaning, the RewriteRule directives beneath it are not going to be processed in the current round (the inner loop).

If the URI changed L will re-inject into the the next round (the outer loop) as the following technical details flowchart shows:

To highlight where the L flag kicks in and where the INTERNAL REDIRECT happens, this is the same graphic with some annotations for your specfic (first) URI rewrite:

It shows that the L flag only exits the inner loop but if the URI has been rewritten (changed) – as in your case – the outer loop takes care that the changed URI will be passed again to all your rewrite rules.

Instead you might want to formulate a condition as the following example from that part of the manual shows:

RewriteBase /
RewriteCond %{REQUEST_URI} !=/index.php
RewriteRule ^(.*) /index.php?req=$1 [L,PT]

(PT has it’s own manual entry, is is more or less not part of the solution, just noting because I quoted the example as-is)

What you actually want to use is the END flag:

RewriteRule css css.php [END,NC]

However contact your system administrator if you have the needed apache version for it (Available in 2.3.9 and later). If not, you need to operate with RewriteCond.