Some anchors in non-spacing marks are not interpolated, fixed to the default master's position

I ran into this situation where the `_bottom` and `_top` anchors are fixed to the default master's anchor position. I have three masters – Thin (100), Regular (400), Black (900) – that the 400 is the default, and I want fontmake to interpolate other instances (100 to 900) using the provided .ufo master files.

```
fontmake -m TestBox.designspace -i -o otf
```

Here is the anchor in the original Thin master:

And this the result of interpolated instances ufo (it's the position from master 400):

Not sure if I did anything wrong with the .ufo or .designspace file.

[TestBox.zip](https://github.com/googlefonts/fontmake/files/8211372/TestBox.zip)