Custom Fonts: Font-weight not applied correctly

The Headings block has a font-weight setting, but it doesn't seem to be applied correctly.

### Steps to reproduce the behavior

1. Edit a post or a page. Add a Heading and a paragraph.
2. Go to Global Styles and select system font for the headings. Click Publish to save the new font pairing.
3. Click on the Heading block. In the block settings change the font weight to Light. Note that the selected font weight is being applied in the editor.
2. Go to Global styles, and select a different font for the headings, for example, Work Sans, Montserrat, or Roboto (all three fonts offer multiple weights on Google Fonts).
3. Click Publish to save the new font pairing.
4. Click on the Heading block. In the block settings change the font weight to Light. Note that the selected font-weight is not being applied in the editor. Try out different weights. The only ones that seem to work are Regular and Bold.

#### What I expected to happen
I expected the font-weight setting to be applied for fonts that do offer multiple weights.

#### What actually happened
I didn't test all the fonts we offer, but from my experiments, the font-weight setting only works with the System Font.

#### Is this specific to the applied theme? Which one?
I don't think so.

#### Video:
https://cloudup.com/cJ9lTGjdnrA