I think I’ve got a case of axes shadowing. Only two axes are defined in the custom parameter’s window. It shows correctly in the Axes Coordinates. But something is wrong with the rendering in the Instance Slider or the Variable Font Preview scripts. Is it a bug or am I missing something?
Variable Font Preview is still interpreting the axis in the “old style” (pre Glyphs 2.5), with fixed names for the axes. It does not know about custom axes.