Ground chuck meatballs are stil pink in middle after being pan-fried and placed in the oven

If during frying the meat balls or nuggest are pressed occasionally to squeeze out watery juice from inside then hot oil will panetrate inside the meat balls which will cook inside of the meatball........

Check the internal temperature and not the color of the meat to test for doneness.

A chemical called Sodium Nitrite is often added to ground and cured meats to retain the meat's pink colour, even when cooked. The use of this chemical is regulated but quite common, and generally believed to be safe. That might have something to do with it...