Perform CSV export (or any kind of export) sends request to Jira. This request waits until the request is finished. 

In case of large number of issues, LB can timeout before the request is finished. Increasing LB timeout to very large value is not a feasible approach.

A better asynchronous design would make more sense where the request to export issues is submitted and initiating request is marked as completed and then Jira perform the action behind the scenes and once the requested export is ready, a download link is provided to the user. In this way, we can avoid LB to timeout.
Possibly an experimental feature behind a dark feature flag can be provided for this kind of export mechanism.

Workaround: A possible workaround is to bypass the reverse proxy and access Jira via http://<host>:<port> method and then export the issues.