Steps to reproduce the issue

1. Start from a site running Jetpack and the Gutenberg plugin
2. Go to Jetpack > Settings > Discussion and enable Subscriptions
3. Go to Posts > Add New
4. Add a Subscriptions block
5. In the block sidebar, attempt to change the font size; it won't work.

https://videopress.com/v/vwMbJPOK

It seems to be limited to installs of Gutenberg so far.

Reported here:

p8oabR-B4-p2#comment-4843