rewrite .* /autodiscover/autodiscover.php last;
instead. The last flag would force nginx to break current location processing and seach for the new one according to the rewrited URI.
You can check complete rewrite directive syntax here.
If you want to discard the DNS issues, start monitoring the IP instead of the domain.
If you are monitoring the domain instead of the IP you are not monitoring directly the Instance, and there could be a lot of factors involved.
Also, it seems that the only way to monitor that has Better Uptime is through ping, and maybe port scanning but I'm not sure, they ...