If you create a new project and deploy pods immediately (e.g. in shell script), the sidecar doesn't get injected.
This is caused by a race between the operator adding the maistra.io/member-of label to the namespace and the creation of the pods. The operator usually loses the race, requiring the pods to be deleted and recreated.
The operator should install a mutating webhook for namespaces that will add the label synchronously during the creation of the namespace. The webhook policy can be configured not to fail if the webhook isn't accessible.