I’m leaning toward authentic as well, I bought a few signed autograph cards from the GHRR auction last year (that was the auction that Madonna tried to stop where her former art dealer/friend consigned quite a few of her personal items), this graph is definitely in line with those.
My personal opinion on the OP is that it is authentic. When it comes to Madonna I want to make double sure. I have come to learn that there are some very good forgeries out there. I would say that 85-90% of the Madonna autographs I look at are not authentic. She is a tough signer especially early in her career.