purchase event not being sent on successful webform submission

We had about the same issue as you described. We found that a redirect prevented the collection of the events. So by making the event handler add a delayed event de event now gets collected when loading the next page. However. The transformer in de purchase event was private. This prevented the transformer from being serialised. By changing it to protected this now works. Not sure if it completely fixes the issue you are describing but maybe it can help.

Nice one @adebruin, I've added a similar patch for sign_up and login events here: https://www.drupal.org/project/google_tag/issues/3406422#comment-15401008 🐛 Sign Up and Login events do not work Needs review

Just to say I have seen this in a couple of other places, and this helped resolve most uses of `addDelayedEvent` I have seen.

The patch above didn't help for me.
I've implemented getData() method for the WebformPurchase event, and after that event works correctly.