It does happen sometimes, but usually with young actors who had only supporting parts before, and then got the lead part in a BL drama. For instance one of the leads from one of the best known Korean BLs To My Star had a few supporting parts before and after he was the lead in a BL. The same with The King Loves actor. He had a few supporting parts after being in a BL, too. I think that maybe it is a career advancement for them, since most of them wouldn’t get a lead role in the mainstream drama for a long time probably.