There was a domain used by Plesk Client or Plesk domain subscription. Plesk subscription is Deleted but the domain is Registered and its Domain Registration subscription is Active.
That is expected behaviour of OBAS, for despite Domain Registration and Plesk subscriptions can be created simultaneously they are managed separately. Plesk subscription is an entity in Plesk and OBAS. It is between OBAS Provider or Reseller and his or her customers whether to renew it or delete.
At the same time, domains are created and managed by a Registrar like Ascio or OpenProvider. It requires an explicit request to a Registrar to terminate a domain. Which is not done automatically upon termination of a Plesk subscription that currently uses the domain because domain usage in OBAS is flexible and a domain can be switched between subscriptions or left unused for some time.