It will not be fixed in Mach3, it is however fixed for Mach4, or at least it is supposed to be, Brian sent me a screenshot showing front/rear toolpaths being used, see below.
Anyway you have two options, either change the code manually to negative X values or you can call a G51x-2 (or G51X-1 if using Radius mode) before the first rear tool is called then after the rear tool moves are finished you cancel with a G50.
Hood