I'll bet you created the environment with Elastic Beanstalk, whose job it is to (among other things) replace instances if they fail.
When you stopped the instance in EC2, EB did its job and created a new instance.
You eventually figured this out and killed the EB env, and the instances stopped reappearing.
But the Elastic IP address assigned to your EB env is still on your account, and it's no longer free of charge, because you don't have a running instance.
So you will be billed about a €/mo until you delete the Elastic IP reservation.
This is ridiculously confusing, and ridiculously common.
When you stopped the instance in EC2, EB did its job and created a new instance.
You eventually figured this out and killed the EB env, and the instances stopped reappearing.
But the Elastic IP address assigned to your EB env is still on your account, and it's no longer free of charge, because you don't have a running instance.
So you will be billed about a €/mo until you delete the Elastic IP reservation.
This is ridiculously confusing, and ridiculously common.