ftp_nlist returns false on file

Question

As the title says, ftp-nlist returns false on files. On directories, it works fine...

First my code:

<?php

$ftp_server = "ftp.myserver.com";
$ftp_user_name = "myusername";
$ftp_user_pass = "mypassword";

$ftp = ftp_connect($ftp_server);
$login_result = ftp_login($ftp, $ftp_user_name, $ftp_user_pass);

// check connection
if ((!$ftp) || (!$login_result)) {
    echo "FTP connection has failed!\n";
    exit;
} else {
    echo "Connected.\n";
}
ftp_pasv( $ftp, true );

$list = ftp_nlist( $ftp, '/' );
if( empty( $list ) ) {
  echo "File list is empty.\n";

} else {
  foreach( $list as $file ) {
    echo "$file\n";
  }

  $list = ftp_nlist( $ftp, '/ftptest.php' );
  if( ! $list ) {
    echo "\nNot found.\n";
  } else {
    echo "\nFound!\n";
  }


}

// close the FTP connection
ftp_close($ftp);
?>

As you can see, I already set passive mode. The output is:

Connected.
/.
/..
/htdocs
/cgi-bin
/logs
/ftptest.php

Not found.

ftptest.php is a file (and actually the php script I'm executing, but it fails on any file).

This behavior started immediately after I upgraded from Debian 10 (Buster) to Debian 11 (Bullseye). What could be causing this?