So, I've run through the design adjusting the sugar addition from boil to primary to secondary and see the same issue you have stated. The OG responds to sugar adds for the boil and primary additions, but does not for the secondary addition.
I can only guess here that since adding sugar to the secondary is pretty rare (since you have taken the beer off the yeast and any significant addition of sugar will overload the yeast still in suspension). I think that the idea of the sugar addition after primary is set up for back sweetening of meads, that the software is not set up to recognize it for alcohol content calculations. Since I don't make meads and have done only a little reading on making them, I am really speculating here. Maybe someone with a better understanding can clarify further.