Shouldn't these alignments (rotated 90 degrees about XPLUS, vs about XMINUS) be completely different?
Yes, they should be different. I am assuming PC-DMIS follows some rules such as ccw=positive for rotation signs so, in that case, rotating by a positive angle around axis <-1,0,0> (XMINUS) should be in the opposite direction as rotating by a positive angle around axis <1,0,0> (XPLUS). This has nothing to do with the workplane or anything else as this is a 3D function.
Right or wrong this quirk will never be fixed. There are too many programs with rotation commands in them that will suddenly break if this is changed. I noticed other quirky commands such as the cross product function so that is not the only one.
