In reverse proxy, when an upstream causes an error (e.g. closes the connection abruptly) the next upstream is contacted. That is not logged but since it is an error it should be logged with ERROR status as it is crucial to help debug our upstreams. The only relevant log I've found is:

ngx_log_debug1(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,"http next upstream, %xi", ft_type);

in ngx_http_upstream_next().

Sincei it is prohibiting to run nginx in debug mode in production for long periods, please bump log_debug to somthing higher. After all it is actually an error, more so than 5xx.
Kind Regards,
-Spiros