You can’t modify service-linked role for Auto Scaling:
With the AWSServiceRoleForAutoScalingPlans_EC2AutoScaling role created by AWS Auto Scaling, you can edit only its description and not its permissions.
However, the KMS permissions should be added to your instance role, not service-linked role for Auto Scaling. So you have to change the role associated with your AWS::IAM::InstanceProfile.
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