Application Load Balancer with AWS Elastic Beanstalk - Target Group
Solution 1:
This was a while back, and what seems to have been working well for me is adding the applicable Target Group under the Target Group section under the Auto Scaling Groups.
So go to EC2 Console, then at the very bottom of the left menu (currently for me anyways) there's the Auto Scaling Groups. Click on the auto scaling group from the list, and on the Details tab, click Edit, and then add the applicable Target Group.
This then seems to keep them in sync even as the instances change.
So it should look like this: Screenshot