Voila websocket stalls for around a minute over DNS URL

I am running voila dashboard which uses Python Plotly framework and apache httpd on the Linux server in dev on a single server.

When I setup the dns URL e.g. (https) dev-myserver-com-/myapp/voila → (https)-dev-vm-abc12345.com:8443/myapp/voila → (http)-localhost:8866 the dashboard starts loading normally but then the whole page hangs for at least 90+ seconds.

I tested on Chrome and Edge but it didn’t make any difference. The browser network tab shows the 101 request getting stalled for 40+ seconds causing the overall delay. If I access without dns i.e. via - https://dev-vm-abc12345.com:8443/myapp/voila URL, this does not happen and page loads in under 20 seconds.

The same issue occurs in a higher test environment where I have DNS with load balancer with 2 nodes hosting voila & httpd.

Would really appreciate any advice/suggestion. Many thanks!