aws auto scaling routing

What load balancer are you using? Does it have sticky sessions enabled by any chance?

Some insights - https://aws.amazon.com/premiumsupport/knowledge-center/elb-fix-unequal-traffic-routing/

Sample Tutorial just for you to double check the configs - https://docs.aws.amazon.com/autoscaling/ec2/userguide/tutorial-ec2-auto-scaling-load-balancer.html

Round Robin means that 2 targets on the ALB will each get every other request. If you want the majority of the requests to go to the new instance (until they're evened out) then you should use Least Outstanding Requests: https://aws.amazon.com/about-aws/whats-new/2019/11/application-load-balancer-now-supports-least-outstanding-requests-algorithm-for-load-balancing-requests/

However, you should also look into why the first instance is crashing when the CPU is only around 40%. See if there's a different resource other than CPU that's your bottleneck (memory, disk, network, artificial application limits, etc). If might be that you can get much better performance by changing instance families to one better suited for your resource requirements