DH doesn’t prevent you from putting anything you want on their nameservers, regardless of whether you own the domain or not.
So if you or someone else already added this domain to their account, but didn’t register it yet, it will show up as available on all Whois pages, but DH will tell you it’s taken.
First step is to check that you didn’t already add the domain to your previous plan. Second is to ask DH support to check the domain.
Personally, I think there should be some mechanism in place to prevent this - it can be very annoying.