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You need a default action on the HTTPS listener too, don't you? I've never used the awsx load balancer, but with the aws API, you need to provide this sort of thing for the HTTPS default aciton:

defaultActions: [
          {
            type: "forward",
            targetGroupArn: defaultTargetGroup.arn,
          },
        ],

Since the awsx code creates the default target group, maybe that's not needed for you?