499 when service application workload upgrade

I have 3 workloads: nginx syslog-ng and my api service application.
everytime i upgrade api service application workload after any modifications, there’s always some 499 status code in response from nginx log. (not all of api requests return 499, most of which is 200)
After a minite or two, the api service goes nomal, no more 499.

I wonder if the container which api service in is killed api service no matter if the request is handled or not.

Thank you for helping me out. Below is config of my api service application wordload.

api service application workload type: Scalable deployment
upgrade policy:

after i change my api server from ip:port to unix sock, the service goes well no matter which upgrade.