Bad sidebearings on composite glyphs of downloaded instances

[justinpenner]

To reproduce:

1. Fonts > EB Garamond (as an example; problem is the same for other fonts too)
2. Instances > click download button for any instance
3. Glyphs made from components have large negative values on their right sidebearings

It seems that some composite glyphs have zero advance width, while others are small but not zero.