Thank you for this bug report. To properly diagnose the problem, we
need a short but complete example script to be able to reproduce
this bug ourselves. 

A proper reproducing script starts with <?php and ends with ?>,
is max. 10-20 lines long and does not require any external 
resources such as databases, etc. If the script requires a 
database to demonstrate the issue, please make sure it creates 
all necessary tables, stored procedures etc.

Please avoid embedding huge scripts into the report.

But did you know that \s will match newline characters?

<?php
$buffer = "
%target:admin_media.php%
%location:%
$showmedia_url = getURL( 'showmedia', 1 );
%end:%
%insert:after%



// ****** following line added by Showfolio mod ******
$showfolio_url = getURL( 'showfolio', 1 );



%end:%

";

// STANDARDIZE LINE ENDINGS
$buffer = preg_replace( "#(?:\r\n|\r\r\n|[\r\n])#", "\r\n", $buffer );

// CREATE WORKING TABLE
$parts = preg_split( "#^\s*%(\w+):#m", $buffer, NULL, PREG_SPLIT_OFFSET_CAPTURE|PREG_SPLIT_DELIM_CAPTURE );

print_r( $parts );
?>

https://3v4l.org/jKupd

The output looks correct to me. What are you expecting to see?

No feedback was provided. The bug is being suspended because
we assume that you are no longer experiencing the problem.
If this is not the case and you are able to provide the
information that was requested earlier, please do so and
change the status of the bug back to "Re-Opened". Thank you.