Not at all unusual.
I have a friend who works in the pit of one of the MGM properties, not MGM itself, who told me that different players, playing the same action get rated differently and earn different comps, and points accordingly.
In other words, three players all play the same action and time.
Player A, a poor player, who plays some basic strategy, but doesn't hit some of his stiffs vs dealer 10's, nor his 12's vs 2 or 3, nor A-7 vs dealer ten.
Player B is perfect basic strategy player.
Player C suspected of being AP.
Player A will earn much more comps for the same action, Player B, less, and player C even less or maybe none bat all.
There are some paces in Vegas, And MGM stores are among them, that reduced comps and offers or maybe none at all, is the first sign of trouble. It is pre-backoff heat, so to speak. I suspect that is what you are experiencing. Should additional heat follow, it may be time to give up playing rated and the benefits of doing so.