Variable Font with 3 axes

Hi, I am getting very strange results with a variable font test.
I created a font with only one glyph. I choose the letter ‘O’ thinking that would be easy enough.
And I created 8 masters to place at the extremes of the 3 axes.
I am trying to interpolate them but I am getting some interpolation errors.
Some outlines are flipping and a times the glyph size is changing.

Can you send me the file?