AWS Capacity Provider for ECS cluster not triggering scale-in event - amazon-web-services

Question

I have added a Capacity Provider to an ECS cluster. While scale-out events work as expected due to changes in CapacityProviderReservation metric, scale-in events do not work.
In my case, the TargetCapacity property is set to 90, but looking at CloudWatch the average for the CapacityProviderReservation metric currently sits at 50%. This has been the case for the last 16 hours.
According to AWS's own documentation, scale-in events occur -
When using dynamic scaling policies and the size of the group decreases as a result of changes in a metric's value
So it seems like the Capacity Provider is not changing the desired size of the ASG as expected.
Am I missing something here, or do capacity providers tied to ASG's simply not work both ways?
ASG and Capacity Provider resources in CloudFormation
Resources:
AutoScalingGroup:
Type: AWS::AutoScaling::AutoScalingGroup
Properties:
AutoScalingGroupName: !Sub ${ResourceNamePrefix}-asg
VPCZoneIdentifier:
- !Ref PrivateSubnetAId
LaunchTemplate:
LaunchTemplateId: !Ref Ec2LaunchTemplate
Version: !GetAtt Ec2LaunchTemplate.LatestVersionNumber
MinSize: 0
MaxSize: 3
DesiredCapacity: 1
EcsCapacityProvider:
Type: AWS::ECS::CapacityProvider
Properties:
Name: !Sub ${ResourceNamePrefix}-ecs-capacity-provider
AutoScalingGroupProvider:
AutoScalingGroupArn: !Ref AutoScalingGroup
ManagedScaling:
Status: ENABLED
TargetCapacity: 90
ManagedTerminationProtection: DISABLED
Dynamic scaling policy for ASG
Current status of the CapacityProviderReservation metric
The CapacityProviderReservation metric has been at 50% for well over 12 hours.
Current status of the Capacity Provider
As you can see, the desired size is still 2, while it is expected that this should have dropped back to 1.
Update
After deleting and recreating the cluster, I notice that the Capacity Provider changes the DesiredCapacity to 2 instantly, even though there are no tasks running.