It ought to be possible to identify the source of the "not smoothness" inherent in the combination of GOG fine wheel and track standards, and illustrate it diagrammatically ....... which I've never seen done.
How about this:
This is a view looking towards the nose of the vee, from just in front of it.
As you can see modern wheels have nothing to support them until they reach the vee, and can fall into the gap between the wing rails with a bump.
The original wheels for 32mm track standards (0-Fine) were wider, sufficient to remain running on the wing rail until they reached the vee.
The narrower modern wheels (3.5mm wide) were introduced by Slater's a good few years ago now, and many other firms have now copied them (because they look nicer, fit inside splashers, etc.)
The solution in 0-MF is to make the flangeway gaps each side of the vee narrower, so that the modern wheels are supported on the wing rails all the way. If you make the flangeways narrower, and don't want to change the wheels back-to-back, it is necessary to reduce the track gauge. Hence the change to 31.5mm instead of 32mm for 0-MF.
A small change in the flangeway gap (from 1.75mm to 1.5mm) makes a big difference because the effect is doubled in the total gap between the wing rails.
p.s. real wing rails are inclined inwards, the diagram is simplified for clarity.