What is likely causing this issue?
Auto Scaling groups can create new instances in a single AZ only.
The EC2 instances have not been manually associated to the ALB.
The ALB should be replaced with a Network Load Balancer (NLB).
The new AZ has not been added to the ALB.
Explanations:
Auto Scaling groups can create instances across multiple AZs if the AZs are enabled in the group configuration.
EC2 instances should automatically register with the ALB when launched in an Auto Scaling group, provided the correct target group is configured.
A Network Load Balancer (NLB) is typically used for different use cases, such as low latency and high throughput. An ALB is sufficient for scaling and handling traffic across multiple AZs.
If the new AZ is not added to the ALB, instances in that AZ will not be registered with the load balancer, and thus won’t receive traffic.