Having run a service on Fargate in prod before I can almost guarantee that the errors he was seeing were 504s which are caused by the server code and the ALB not having their request-timeouts set to to the same value.
It’s my understanding that you want to set the timeout in the server code to be higher than the idle connection timeout in the ALB. With them set to the same value, timeout errors will look different depending on whether the application or the ALB dropped the connection.