Why is my instance failing ELB health checks when adding it to the load balancer via Ansible?

I am trying to add an EC2 instance to an Elasitic Load Balancer using an Ansible playbook, with the ec2_elb module. This is the task that should do this:

- name: "Add host to load balancer {{ load_balancer_name }}"
  sudo: false
  local_action:
    module: ec2_elb
    state: present
    wait: true
    region: "{{ region }}"
    ec2_elbs: ['{{ load_balancer_name }}']
    instance_id: "{{ ec2_id }}"

However, it routinely fails, with this output (verbosity turned up):

TASK: [Add host to load balancer ApiELB-staging] ****************************** 
<127.0.0.1> REMOTE_MODULE ec2_elb region=us-east-1 state=present instance_id=i-eb7e0cc7
<127.0.0.1> EXEC ['/bin/sh', '-c', 'mkdir -p $HOME/.ansible/tmp/ansible-tmp-1409156786.81-113716163813868 && chmod a+rx $HOME/.ansible/tmp/ansible-tmp-1409156786.81-113716163813868 && echo $HOME/.ansible/tmp/ansible-tmp-1409156786.81-113716163813868']
<127.0.0.1> PUT /var/folders/d4/17fw96k107d5kbck6fb2__vc0000gn/T/tmpki4HPF TO /Users/pkaeding/.ansible/tmp/ansible-tmp-1409156786.81-113716163813868/ec2_elb
<127.0.0.1> EXEC ['/bin/sh', '-c', u'LANG=en_US.UTF-8 LC_CTYPE=en_US.UTF-8 /usr/bin/python /Users/pkaeding/.ansible/tmp/ansible-tmp-1409156786.81-113716163813868/ec2_elb; rm -rf /Users/pkaeding/.ansible/tmp/ansible-tmp-1409156786.81-113716163813868/ >/dev/null 2>&1']
failed: [10.0.115.149 -> 127.0.0.1] => {"failed": true}
msg: The instance i-eb7e0cc7 could not be put in service on LoadBalancer:ApiELB-staging. Reason: Instance has not passed the configured HealthyThreshold number of health checks consecutively.

FATAL: all hosts have already failed -- aborting

I have my ELB configuration defined like this (also via Ansible):

- name: "Ensure load balancer exists: {{ load_balancer_name }}"
  sudo: false
  local_action:
    module: ec2_elb_lb
    name: "{{ load_balancer_name }}"
    state: present
    region: "{{ region }}"
    subnets: "{{ vpc_public_subnet_ids }}"
    listeners:
      - protocol: https
        load_balancer_port: 443
        instance_protocol: http
        instance_port: 8888
        ssl_certificate_id: "{{ ssl_cert }}"
    health_check:
        ping_protocol: http # options are http, https, ssl, tcp
        ping_port: 8888
        ping_path: "/internal/v1/status"
        response_timeout: 5 # seconds
        interval: 30 # seconds
        unhealthy_threshold: 10
        healthy_threshold: 10
  register: apilb

When I access the status resource from either my laptop or from the server itself (as localhost) I get a 200 response as expected. I also added a command task to the Ansible playbook, right before adding the instance to the ELB, to confirm that the application is booted up and serving requests properly (and it is):

- command: /usr/bin/curl -v --fail http://localhost:8888/internal/v1/status

I don't see any non-200 responses for the status check resource in the logs for my application (but of course, if the requests never made it as far as my application, they would not be logged).

The other weird thing is that the instance does get added to the ELB, and it seems to work properly. So I know that at some point, at least, the load balancer can access the application properly (for both the status check resource, and other resources). The AWS console shows the instance is healthy, and the Cloudwatch charts don't show any failed health checks.

Any ideas?