There are a few suspicious cases but given how human immunity system works, it is very very unlikely that it is due to reinfection (i.e. a cured patient getting

There are a few suspicious cases but given how human immunity system works, it is very very unlikely that it is due to reinfection (i.e. a cured patient getting infected again). A far more likely explanation is either poor testing that gave a false "all clear" while the patient was still sick. The tests are not 100%, that you don't detect the virus in the sample doesn't mean that the patient is not infected - it only means your sample didn't contain the virus. Which could happen for all sorts of reasons. That's why multiple tests are usually required - commonly two, in some cases up to eight consecutive tests had to be clean before the patient was discharged. Or the patient has not been completely treated and the virus became dormant for a while, reappearing again later. This is known to happen with other diseases as well - e.g. ebola virus could stay dormant for weeks in the eyes and cause the disease to return again.