Hook postProcess doesn't work with message templates form
Yes, some admin config forms doesn't invoke postProcess hook. Try below patch to invoke post process hook
diff --git a/CRM/Admin/Form/MessageTemplates.php b/CRM/Admin/Form/MessageTemplates.php
index 5f809f2..67cac30 100644
--- a/CRM/Admin/Form/MessageTemplates.php
+++ b/CRM/Admin/Form/MessageTemplates.php
@@ -309,6 +309,7 @@ class CRM_Admin_Form_MessageTemplates extends CRM_Core_Form {
       }
 
       $messageTemplate = MessageTemplate::save()-&gt;setDefaults($params)-&gt;setRecords([['id' =&gt; $this-&gt;_id]])-&gt;execute()-&gt;first();
+      $this-&gt;postProcessHook();
       CRM_Core_Session::setStatus(ts('The Message Template \'%1\' has been saved.', [1 =&gt; $messageTemplate['msg_title']]), ts('Saved'), 'success');
 
       if (isset($this-&gt;_submitValues['_qf_MessageTemplates_upload'])) {