Aux busses are and have always been 48-bit internally. In other words all summing on the bus happens at 48-bits, unless the summing had to span a DSP. With the new mixer (PT v5.3.1), the internal resolution of the Aux bus will always stay at 48-bits even when spanning DSP's. The output of the bus after summing then returns to 24-bit, which is why the dithered mixer is useful.