Went to the dealership today to have it checked out, and it turns out it is a common issue with new NIU scooters. He had taken apart multiple rear brakes from various NIU scooters with the same issue and there was no fix. Also, the front wheel was a little bit loose which caused that the magnet on the front brake read the wheel distance wrong and cause a weird braking sound. Poor assembly I guess? But they got that fixed.
Hope this helps some people with the same questions.