Here is our definition of an 'Unknown' cluster status:
This is a situation where either there was no event running on the cluster or we failed to collect any information about the event.
To expand on this, clusters can show up without event data. That’s because the cluster will still have an instance (aka available metrics) and potentially tasks (aka long running tasks). That's why you can see that 'Unknown' ECS cluster showing data on the ECS Resource, and ECS Allocation reports. The clusters can still be 'Unknown', however, because there is no start/stop event. If there is no start/stop event, there is no event data.