Put an imported registrant back on the public guest list

Open Events → Events → select your event → Registrations.

When the event's guest list is enabled and the administrator has edit access, open the registration, edit registrant information, clear Exclude name(s) from the public Guest List, and save. The checkbox is conditional on the event showing its guest list, so first check that event setting if the control is absent. Confirm the resulting public list without changing attendance or creating a duplicate registration.