Custom Tag Not Populated In Auto-Repsonder Email

User signup to our site, we do followings

1) Send confirmation email (using 3rd party api - not via mailwizz)
2) From mailwizz api, add user as "unconfirmed" in mailwizz with all user data (firstname, etc)

When user confirm the email

Then we call mailwizz api to update user's status as "subscribed"/"confirmed".
Here we have put auto-reponder which send welcome email.

In that auto-responder we added a custom tag - [FIRST_NAME].
But when I received the email, firstname is not populated. Whereas in subject line firstname is populated.
In email body, it is still [FIRST_NAME]. Remember that user's data is already added way before sending auto-reponder.1595358832882.png

Also in "Available Tags", its showing all custom tags including [FIRST_NAME]

BUT when I send test email to same user, the [FIRST_NAME] is populated like


I am not sure how to fix this.