Hi,
There is always a delay between issuing a request to update the desired instance count, and the ASG reflecting the counts. So you may have been in the waiting period where the ASG hadn’t detected the updated desired instance count yet.
You can have terraform wait for the desired count to be reflected by configuring the capacity wait configurations: https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/autoscaling_group#waiting-for-capacity
Hope this helps!
Best regards,
Yori