By default Puppet doesn't follow symlinks when updating files so with /etc/pam.d/system-auth being a symlink to /etc/pam.d/system-auth-ac it wasn't being updated. Added 'links => follow' and this now works as expected.

Author: erwbgy

Modulefile

@@ -1,5 +1,5 @@
 name    'erwbgy-pamldap'
-version '0.1.0'
+version '0.1.1'
 source 'https://github.com/erwbgy/puppet-pamldap.git'
 author 'erwbgy'
 license 'Apache License, Version 2.0'

manifests/config.pp

@@ -8,6 +8,7 @@
   File {
     owner => 'root',
     group => 'root',
+    links => 'follow',
   }
   file { '/etc/pam.d/system-auth':
     ensure  => present,