A17 changed how treatment works on diseases, previously it was never possible to reduce the severity of a disease, you could only slow the severity gain down, now when you treat a disease it also decreases the severity. For most diseases this treated severity reduction is less than the severity gain, so the severity will still increase, for malaria the severityPerDayTended is -0.20 while the severityPerDayNotImmune is 0.137, so the severity will drop while being treated and can dip below the visible threshold for the disease, causing it to become hidden again, this is the oversight. The disease is still there, the pawn is still gaining immunity, but without further treatments it will increase in severity again until it unhides and gives you the disease warning again.