Uvicorn not processing some requests randomly - nginx

Question

We are running a Fastapi + Uvicorn web application using gunicorn as a process manager and Nginx as the reverse proxy server. The application is running in async mode for most of the i/o operations (DB call, Rest apis). The whole setup is running inside a Docker container on Ubuntu 16.04.
The setup works most of the times but sometimes it does not process a request at all & it gets timed out at Nginx end. We also tried taking Nginx out of the setup and observed that few requests get processed after really long time (like after 15 mins). This is very random but usually happens 2-3 times in an hour.
Below is the gunicorn config that we are using –
host = os.getenv("HOST", "0.0.0.0")
port = os.getenv("PORT", "80")
# Gunicorn config variables
workers = web_concurrency
bind = f"{host}:{port}"
keepalive = 2
timeout = 60
graceful_timeout = 30
threads = 2
worker_tmp_dir = "/dev/shm"
# Logging mechanism
capture_output = True
loglevel = os.getenv("LOG_LEVEL", "debug")
And gunicorn is invoked with command exec gunicorn -k uvicorn.workers.UvicornWorker -c "$GUNICORN_CONF" "$APP_MODULE"
We have tried several config changes like –
Changing the number of workers, worker timeout
Changing the process manager from gunicorn to supervisord
Offloading the CPU intensive task to Celery instead of threading
Binding uvicorn app to unix socket instead of proxy server