Envoy returns 200 directly without waiting for the end of the request

Hi community,
I am running a server which using the envoy as a api gateway to convert the http request to grpc and distribute it to the service.
It was a smooth start,but recently I found some problems.I found envoy aways return 200 without waiting for the end of the request.
for example，I added sleep time to my code，the server did return response while sleeping but envoy returned 200 whith empty response.
envoy access.log:
{"url":"user/login","method":"POST","upstream-service-time":"4","my_custom_header":"Bearer *************************","duration":5,","status":200,"x-forwarded-for":null,"respflags":"UPE","bytes-sent":0,""protocol":"HTTP/1.1"}
You can see status is 200 but bytes-sent is 0