This feature must not belong to original slab floor/opus quadratum building phase since it is constructed in a rather rough cut of wall 5664/5665.
Speculated to be either a water or fire feature due to linear cuts in the bedrock leading into the arch and the arch's proximity to other water features (e.g. drain 5600, associated cistern under room 16) or due to a substantial layer of what seems to be burnt/reddened soil and layer of charcoal under the arch as well as its proximity to the possible hearth/kiln 5720.
Brick arch between 5664 and 5665 (5757), Broken-tufo slab and small rock below 5757 (5758), Rectangular cut in bedrock into slab floor 5726 (5759), and nearby walls 5664 and 5665