Anything on the AWS side?

Can you look at the logs there and see what's up?

can you link to the code that is creating the service?

let httpBackendService = new awsx.ecs.FargateService("http-backend", {
    cluster: cluster,
    taskDefinitionArgs: {
        containers: {
            httpBackend: {
                image: `<http://734247230719.dkr.ecr.us-west-2.amazonaws.com/compute/app-ions:${appIonsImageTag}|734247230719.dkr.ecr.us-west-2.amazonaws.com/compute/app-ions:${appIonsImageTag}>`,
                portMappings: [httpBackendListener],
                cpu: 1024,
                memory: 2048,
                environment: [
                    {
                        name: "COMPUTE_CONFIG_URI",
                        value: configUri
                    },
                    {
                        name: "NEW_RELIC_APP_NAME",
                        value: "name"
                    },
                    {
                        name: "NEW_RELIC_LICENSE_KEY",
                        value: "..."
                    },
                    {
                        name: "NEW_RELIC_ENVIRONMENT",
                        value: env
                    }
                ]
            }
        }
    },
    // the app seems to take 4 mins to start up
    healthCheckGracePeriodSeconds: 60 * 4,
    desiredCount: 2
});

Which logs?

const httpBackendLb = new awsx.elasticloadbalancingv2.NetworkLoadBalancer("http-backend-lb3", {vpc: vpc});

const httpBackendTg = httpBackendLb.createTargetGroup("http-backend-tg2", {
    port: 8880,
    healthCheck: {
        path: "/_healthcheck",
        // healthyThreshold: 2,
        // unhealthyThreshold: 4
        // matcher: "200"
    }
});

const httpBackendListener = httpBackendTg.createListener("http-backend-listener", {
    port: 8880
});

service http-backend-55db594 has reached a steady state.

service http-backend-55db594 has started 2 tasks

service http-backend-55db594 has reached a steady state.