It's definitely score. The sandbagging thing is the only reason I'm aware of. You tend to take a down slide after moving up a tier, so I can understand the reasoning.
Just curious, when did this bump happen? It happened to my friend a week or two ago. Sounds like the same scenario. I think this happened during an update of the game. (which put it down all night - arrrrr) Maybe the cut-off was adjusted? Maybe it's always adjusting for this very purpose?
All I can say, is be happy with pro. I shot -10 as a pro, and hover around par as a master with the best clubs. I only joined a couple months ago, but so far, I'm happy with the way the game is built. I have the usual gripes, including the swing meter, but hey, this is golf!