It seems the company didn't include it in health insurance.

When a foreigner becomes a workplace subscriber, the employer must report to the National Health Insurance Service within 14 days from that date. You can get the Health Insurance Qualification Certificate yourself from Government24 or the National Health Insurance Service. If you have no records, first check whether the filing deadline has passed and has not yet been reflected in the inquiry. Having no records does not mean you are unenrolled, so ask the National Health Insurance Service at 1577-1000 about the date you obtained your qualification and the status of your report. If it is confirmed that you did not report, ask 1350 to see if this counts as a reason for changing your workplace.