Wrong yScale interpolation in variable TTF font

[Wavefont](https://github.com/a-vis/wavefont/tree/radius-caps) experiment, each bar consists of 3 components:
* top cap
* bottom cap
* value bar

Value bar template is simple 100% height rectangle:
```glif

```
, that is inserted in glyphs and scaled down via `yScale`.

Glyph template is the following:
```glif

```

afdko renders variable OTF the right way:

but fontmake creates artifacts (bar height is more than needed due to wrong interpolation):

It looks like it happens due to reverse interpolation of `yScale` between masters.

Minimal `designspace` setup is here:

[yScale-artifacts.zip](https://github.com/googlefonts/fontmake/files/6055629/yScale-artifacts.zip)