I understand you have issues sending the email, username, and user_id attributes with your Post-User Registration Action script.
First, I have gone ahead and tested the Action and did not find the same observations. When I check the event.user object, I can see the user profile attributes listed here.
Given that, have you made sure that your Database Connection has the Requires Username toggle enabled?
And, could you please try using the Real-time Webtask Logs Extension to see the complete event.user object during the signup flow?
Lastly, I double-checked your Action saved on your tenant, and there seem to have three dots prepended to the event.user which is syntactically incorrect. You’ll want to do something like the following: