Well, I finally started raising a second desynth class, and the drops are far less than I had feared. My GSM went from 77.65 to 77.55 while my BSM went from 1 to 50. Annoying, but hardly game breaking. I somehow doubt this is a bug, and I expect that the higher your total skill points, the more of a loss, until it's one for one at 300 skill.