Not only South Korea, even the Chinese reported similar incidents mid-January itself.
No one knows for sure whether it is a re-infection or just a reactivation during the convalescence period. But it was suggested that most probably these are re-activations.
Usually a patient gets discharged after two consecutive PCR tests become negative. Depending on the severity of the lung infections, it is possible that traces of the virus still remains in the lungs.
There could be false negatives for oropharyngeal or nasopharyngeal swabs tests, affected by the site from which the sample was taken, the experience of the operator, and the actual quantity of virus.
Bronchoalveolar lavage fluid specimen is the MOST accurate. Other samples, like for eg sputum, blood, or stool swabs are far less reliable.
Running multiple tests and collecting different specimens for the RT-PCR would be more effective to maximize sensitivity.
The Chinese recommendations were...
Both nasopharyngeal and oropharyngeal swabs tests to be performed to reduce the false-negative rate. . And more tests from more sites should be considered.
Patients in convalescence should also be
regularly tested for infectivity assessment, and all those discharged should be home quarantined for at least 14 days.
Let the experts find out what the real cause is.