I’m not sure what the problem is. If one assigns a volume shader to the fluid container (like volumeFog) then the maya software renderer would do this … the fluidShape is both a shader and a cube volume shape. By default it is assigned to its self. (unless created as a shader or texture)
However with Arnold, it seems to always treats it as its own shader, and I don’t think one can get that behavior by accidentally assigning a shader to the fluid.
Do you get the problem all the time, for example with just a default fluid, or just with a particular scene?