My mastiff sounds exactly like your Rott, but the mastiff doesn’t give a warning bark or growl, only an alert posture.
The one time I saw it in person, the person got the message while I was initially clueless. We gave an old truck to a young adult who needed transportati0n to work, and after he was in our house a while, maybe half an hour, he said, “Your dog doesn’t like me” and then I noticed that the dog was in the alert posture. He left the house and the dog went back to sleep at my feet.
Mastiffs are a hand full, no denying it. And they don’t give much of a warning, unlike the other breeds.