Why does this work?

cluster_user_name = pulumi.Output.all(mdba_cluster_name).apply( lambda x: x )
pulumi.export( "cluster_user_name", cluster_user_name )

But this:

cluster_user_name = pulumi.Output.all(mdba_cluster_name).apply( lambda x: str( x + "foo" ) )
pulumi.export( "cluster_user_name", cluster_user_name )