Ray Mooers (owner of Dusty Strings) once told me that tuning in C raises the tension on a harp about 10% over a harp tuned in Eb. So by tuning in C, it could be that those strings are now being pulled harder across the bridge pin, causing a closer and tighter connection and less twanging. You could try a slightly higher gauge of string on those two, which would accomplish the same thing.
It has something to do with the angle of the string as it comes off the bridge pin and heads up to the tuning pin. Or at least that how I think of it in my little pea brain.
It is really odd how sometimes twangs, plinks, buzzes, and bangs go away by themselves while I am trying to figure out what action to take. A mystery! I can understand a problem gradually developing, but gradually improving? Almost like the harp can heal itself! Anyway, glad it is getting better for you!