Variable font output: mixed up vertical metrics?

When I try to make a variable font from Cantarell, the default outlines seem to be vertically lower than the interpolated instances, at least in GNOME (no problem in Firefox).

1. Get https://gitlab.gnome.org/GNOME/cantarell-fonts
2. `fontmake -g Cantarell.glyphs -o variable`
3. Use e.g. `ftview 16 Cantarell-VF.ttf` to view and use `n`/`p` to switch between instances. The first is normal, the instances are shifted upwards it seems.

This vanishes if I use the same hhea and typo metrics.