Rounding issue

I have a font built with three masters. Opening the interpolated instances that Glyphs generated in FontLab reveals that although the nodes were all exactly on the defined x-height line in my masters, they vary in height in the generated .otfs. Arrows in this image point to the nodes that came out one unit too low.

This looks like a rounding error, but shouldn’t straight lines at a common y-value all round to the same y-value?
(I have quadruple checked and confirm that all of those nodes are exactly on the mean line in every master.)

What version do you have? I fixed a small issue in the 1.3.8 version. Can you send me the file that I can verify this?