It looks like you are not setting the server.basePath setting or any of the other settings that Kibana provides specifically for use with reverse proxies: Configure Kibana | Kibana Guide [7.12] | Elastic
I found some relevant and helpful information in this other thread: Nginx reverse proxy setup for Kibana
Here's a user who has a working config with basePath: Nginx reverse proxy with rewrite and app/kibana#/discover