Invoice status not getting updated even after the order is successfully paid.

Probably a commerce_invoice issue, not a core issue.

But the onPaid event is not dispatched when the order is placed. that is why I raised the issue in commerce core.

The order is paid event is dispatched only if the order is paid in full and the payment is NOT an authorization (payment with the completed state), is that the case?

In my scenario, the order paid event is dispatched when the total price is zero else it is not being dispatched. I am using commerce braintree for the payment gateway and getting a success response for the same. If the total price is more than zero, the onPaid event is not being dispatched.

Did you check the payment state? If you're only authorizing payments, then the order is not marked as paid until you capture the payment...

The payment state was submitted for settlement in the payment gateway and that is how the braintree is making the API transaction sale.

I'm referring to the payment state in Drupal...

I will check the payment state and get back to you ..