You can try a couple of things to isolate the issue and re-verify the following configuration:
Has routing Production traffic to the Staging slot been configured either automatically from the Deployment slot blade or manually by adding the ‘x-ms-routing-name’ query parameter in the code?
To confirm that the requests are truly not being routed to the Web App in the Staging slot, kindly configure ‘Failed request tracing’ to validate from the application side that the requests are truly not being routed to the Web App in the Staging slot: https://docs.microsoft.com/azure/app-service/troubleshoot-diagnostic-logs#log-detailed-errors
Just to narrow-down the issue, If feasible, restart the WebApp and analyze to see how long the slot does not receive any traffic? When does the WebApp in the slot starts receiving the requests again? Review and capture the timeframe.