This incident has been resolved. Post mortem summary:

What happened?
api.flexpa.com was unavailable and 503s were returning from the load balancer.

Root cause analysis
We tried to deploy a Task Definition that was pointing to secrets that were not in the parameter store, because those secrets were removed after the deploy was initiated.

Action items to prevent its reoccurrence
Ensure that removing a secret from Doppler should regenerate a task definition and redeploy.