*spoilers for Totland arc*
In chapter 849, we see Brook defeat the guards in the treasure room and proceed to copy the poneglyph. Next panel, we see Big Mom outside the room, who comes in to take care of Brook herself. After Big Mom enters, we don't directly see the poneglyph. And in a later chapter (853 I think) on going through Brook's stuff, they don't find any rubbing of the poneglyph. This is a far fetched theory, but I think Brook could have completed taking the rubbing before Big Mom entered, and instead of keeping it inside the container on his back, he kept it inside his skull like he keeps his Tone Dial. I don't think we'll see them go to the treasure room again so they can get a rubbing, so I think Brook has already taken care of that part.
What do you guys think?