So, she didn't consent to ... having butter rubbed on her butt?

I don't think she consented to Brando sticking his fingers against/into her anus.

http://variety.com/2016/film/news/last-tango-in-paris-rape-scene-consensual-bernardo-bertolucci-1201933117/
So she was telling the truth

No, actually that is not correct. This story is not about consent. In essence when she agreed to do the film she agreed to do this scene, as this scene is in the script. And so, signing to be paid to do it she had given her consent.

On the day they were to shoot this scene Brando and Bertolucci had a discussion of the scene over breakfast during which Brando suggested he should work butter into the scene as a lubricant. Maybe he did this with the idea that it would be more "realistic" for him to use a lubricant if he was supposed to be having sudden anal sex with the woman (which was in the script)...this does actually make sense as it would not be easy to surprise someone with anal sex without lubricant.

If you watch this scene in the film, which it seems like many people commenting on this have not done, then you see the butter is introduced to the scene by Brando asking Maria to go to the kitchen and get the butter as he sits on the floor eating.

She brings the butter and throws it at him.

Then she finds a "hiding place" in the floor that causes Brando to crawl toward her to attempt to open the hiding place. She grabs his hand and stops him from opening it because she seems to think doing so would be bad luck.

He then turns his attention to her and asks if he can open her pants/vagina and proceeds to put his hand there and says perhaps there are jewels hidden in it. He then asks her if she is afraid and she says "No." and then there is a shot of Brando looking back for the butter and pulling it toward him with his foot.

Now, I don't know in what sequence they shot each of these set-ups but there are several set-ups that introduce the butter and Brando pulling the butter toward them with his foot, basically as she watches this as she is facing toward where the butter is, and so if this was shot in sequence it is pretty clear he asks her to get the butter, she gets it, he pulls it toward them as he is discussing and attempting to open her pants...so she knows he is bringing butter into the scene for something...and it is obviously not toast. So, the film crew had to set up several shots that involved the butter that led up to the set-up where he simulates anal sex with her...

So, watching it all it seems as if she likely would have known, maybe only told the day of shooting, that butter would be part of the scene because they sure took some time shooting all the different set-ups that introduce the butter.

Now, it is possible they shot the set-ups that introduce the butter after the simulated anal sex scene and when Maria gives him the butter she throws it at him violently...which maybe indicated how mad she was about butter being in the scene.

There really is not a question of "consent" though as the scene was always in the script just sans butter.