The estimate will only be linked to the original invoice you've created, cetanorak.

If the status of the estimate is Closed, it will change back to Accepted once you deleted the original invoice.

As my colleagues mentioned, this issue is only happening to the Airy Classic Invoice template. You'll want to use a different one until this is fully resolved.

Also, I'd encourage you to contact our Customer Support team so that you'll be added on the affected users list and receive updates from time to time. You can follow the steps they've posted above on how to contact them.

Reply to this thread anytime if you have other questions.