Cdns.ovh.net has a domain blacklist?

I've a couple of OVH machines - all are set (by default) to use 213.186.33.99 aka cdns ovh net (removed dots as apparently I can't post links) as the only DNS resolver.

I use one machine as a gateway for email (between local network and an upstream security provider) - but just now I noticed in the postfix logs that a domain: national-lottery.co.uk email in was rejected as domain not found.

Spent some time investigating - and it turns out that if I ask cdns.ovh.net for any NS records relating to the domain, then I get no reply. Using any other resolver, it works just fine.

So: Does OVH operate a domain blacklist on their standard resolver?
I can't find any reference to this.

Of course, I'm now going to no longer use OVH resolvers, but it'd be useful to share this info.

Thanks