Variable-pitch font has stopped rendering correctly

Question

For some reason, variable-pitch fonts have stopped rendering correctly. They previously worked fine, but something seems to have changed (though nothing in my .emacs has changed).

I am using OS X 10.10.3 and Emacs installed thru homebrew. From the startup screen:

This is GNU Emacs 24.5.1 (x86_64-apple-darwin14.3.0, NS apple-appkit-1347.57).

Question: how do I fix this to make variable-pitch work again?

Here are a couple pictures to explain:

The Emacs startup screen uses both fixed-width and variable-width fonts, and the ones that inherit from variable-pitch are rendered incorrectly.

I found a script on EmacsWiki to test several fonts at once. Again, fixed-width fonts render correctly, but variable-width fonts do not.