It’s certainly not uncommon here in NJ. I’ve had many GMC dealers refuse my supplier discount when shopping for my Sierra Denali. Same with Dodge dealers refusing my affiliate code when shopping for my Challenger.
The dealer has the right to refuse the discount. It’s most often done because they feel the next guy will walk in off the street and buy the car at MSRP, which results in higher profit margin for the dealer versus the supplier discount price. Where-as dealers that are OK with the discount are generally more concerned with volume and not lazer focused on the most margin for each vehicle.
If you read the fine print of the GM supplier offer, they can accept the discount but refuse the reduced documentation fee, which is also common. I’ve never been successful getting both the GM discount + fixed doc fee. I have found that it can vary by geography, for example I’ve had much better success driving into PA and getting discounts honored.