I'm a D-2 visa student and my NHIS bill was always 76,000 KRW?

That jump almost always comes from how your insurance category switched. As a D-2 student you normally pay the special reduced foreign-student premium as a regional subscriber (지역가입자), which is the flat amount you were seeing around 76,000 won. During your internship the company enrolled you as a workplace subscriber (직장가입자) and paid your share, which was correct. The problem is that when the internship ended and you reverted to a regional subscriber, NHIS often re-registers the person under the ordinary regional calculation instead of automatically restoring the reduced student rate, and that ordinary rate can be far higher, which is why March suddenly shows 500,000.

The fix is straightforward, but you have to ask for it. Contact the National Health Insurance Service, call 1577-1000 and press for English, or visit your local NHIS branch, and explain that you are a D-2 student and the student reduced premium was not reapplied after your workplace coverage ended. Bring or submit a current certificate of enrollment (재학증명서) and your ARC. They can reassess your account, put you back on the student rate, and correct or refund the overcharged month once the category is fixed. Do it soon, since sorting it before the due date saves you from overpaying and then chasing a refund.