I have confirmed that iron golems are not moving down full blocks or even stairs if their movement is to the north or west (negative x or z axis).  They move fine in the other directions.  This is especially obvious given they protect villages which have many examples of downward stairs that golems get stuck on (see screen shot).  After they walk up, they can't walk back down.

Lastly, FWIW, I am a modder.  I fixed it but it was a big pain in the neck.  It has to do with the golems having a width greater than 1.0 (it's 1.4).  There are two places where entity width is considered, one in path finding and one in path following.  In both places it is hard-coded to always extend the width in the positive X and Z.  Of course this breaks north and west.