I just finished raising our bathroom ceiling back up to 8' using green
board.

This might have been a mistake. Some brands of green board are said to sag over
time. It might be wise to check with USG (or whoever) customer service and see
whether you are going to be OK on this. In past remodels I've found that
greenboard has a lot more voids and bubbles in the gypsum than the regular
drywall. Don't know whether that's the cause or not. HTH

Joe