The short answer is that he must be released, not remanded, and the reasoning rests on treating this as a two-part test.
First, was the arrest itself lawful. Under Article 22(1), an arrested person must be told the grounds of arrest, and the Supreme Court in Prabir Purkayastha v. State (2024) held that under UAPA this information must be given in writing. If it was not, the arrest fails this test and is illegal from the moment it happened.
Second, can an illegal arrest still support a valid remand. It cannot. Remand is meant to extend an existing lawful custody while investigation continues; it is not a mechanism for fixing a custody that was never lawful to begin with. A Magistrate who orders remand without checking whether written grounds were given has not properly applied judicial mind to the legality of the custody before them.
Since the first condition fails here, the second question never gets to a yes. The person cannot be remanded and is entitled to release.
Someone unfamiliar with Prabir Purkayastha might argue that a missing written record is a paperwork lapse the Magistrate can overlook if the person otherwise knew, in general terms, why they were arrested. That argument does not survive the actual reasoning of the Supreme Court, and it is worth explaining why before stating the answer.
The argument for treating it as a curable lapseSo the paperwork argument fails, and the outcome stands: without written grounds of arrest under UAPA, the person cannot be validly remanded and must be released.
\[ \boxed{\text{Release, not remand, because an illegal arrest cannot be cured by a later remand order.}} \]