Nyx's API allows Vanity URLs to be protected for up to 7 days after that URL has been created, in which after that can be either reset or overlapped by someone else freely. If you created the Vanity URL in the first place, you can always renew it at any time by redoing the same command, or you can update the URL that the vanity URL redirects to. After those 7 days since the Vanity was created, anyone can do anything to that URL.
If this isn't the problem you are facing, please join the support server and talk to an admin.