In Grinding and Honing part 4 Bos warns against lapping with diamond because he says the sharper / harder diamonds shred the garnets in the coticule making them less effective for polishing. That would support what you say. Since reading that I have stuck to Silicon Carbide for lapping. I have also started pairing similar sized / types of coticules and rubbing them together after honing until the suction ceases. So far this seems to be quite effective for keeping them flat.