Our experience is that anytime a change of any sort, no matter how small, is made to an existing reservation, the SH OBC drops off. Don't know why this is, just is.
Whenever I'm in contact with my TA or Princess to make a change, I always ask them to check to see if the SH benefit is still on the reservation. If not, I ask them to fix it. This can take some time as fixing this issue is done by another group of people and not the front line reservation people.