I wondered about that myself and decided that they were probably removed by those doing the excavation for conservation purposes. If you refer to the 11th photo

I wondered about that myself and decided that they were probably removed by those doing the excavation for conservation purposes. If you refer to the 11th photo where you are looking diagonally across the room toward the corner you can see evidence in the thin accent tiles that the collapse of the columns along the right wall and left of the corner damaged the floor. Several of those accent tiles are broken as if something long crushed them as it landed. It is especially noticeable in the lower center foreground where there are several crushed tiles. They have taken care to place their post jacks on a surface elevated above the original floor level to avoid disturbing the rest of the layout.