delegation – Microsoft DNS multiple name servers for a domain behavior not documented

A delegated domain on a Microsoft DNS server that delegates a sub domain to external name servers. ex.

subdomain.domain.com
NS IP x
NS IP Y

The advanced tab shows server options and round robin check box. This appears to work as expected to round robin queries between X and Y when a income query requires resolution by the subdomain name servers.

We had heard from Microsoft that they send queries to both name servers at the same time and use the response from the one that responds first. And then prefers the fastest name server of the N name servers.

There is no public documentation that states this from Microsoft.

Testing shows round robin in a lab (latency is the same for each name server) using packet trace and no proof it sends requests to all configured name servers.

Does anyone have experience in this load balance or parallel name server queries.