Following reportbug's instructions, since I don't know exactly what package caused the problem, I'm mailing this list.
I'm running testing and tonight I upgraded as usual. Soon after, the FontAwesome symbols U+f176 (fa-long-arrow-up) and U+f175 (fa-long-arrow-down) stopped rendering properly in some applications. I believe it is gtk related because terminal based applications still render the arrows fine. Opening a file with both arrows in vim works as expected, but opening the same file in gvim makes the arrows fail to draw properly. Has anyone else experienced this issue? Is it confirmed to be a gtk problem? What package should I report it under? James