I just remember nextcloud has trusted_proxies option located in config/config.php file. Not configuring it (or configured it wrong) can cause nextcloud to not trust the HTTP_X_FORWARDED_PROTO header it received (it assumed to be spoofed).

https://docs.nextcloud.com/server/latest/admin_manual/configuration_server/reverse_proxy_configuration.html

I’m not sure what happened in your setup, but here is how https handling should works:

• users request https://example.com to your reverse proxy
• reverse proxy pass the requests to upstream server, but using http instead. It also add HTTP_X_FORWARDED_PROTO header to let the upstream server know that even though the request is over http, the end user actually served over https so it should assume the connection as https
• the upstream server return a response, it take into accounts that the request is already done via https (e.g. generating links with https:// prefix, etc).

I think this chain is broken somewhere in your setup and cause nextcloud to not receive the HTTP_X_FORWARDED_PROTO header from nginx proxy.