Thinking this to be a powerline issue related to powerline/powerline#1494, I originally posted the following at powerline/powerline#1494 (comment)
Thanks so much to Trent Harvey and @ZyX-I for their help working through this. (Twitter thread.)
I just encountered this problem. I'm not sure what causes it, since I've been using my terminal configuration for years without this issue. I did not install or upgrade any software today. Rebooting my machine did not fix it.
I'm running: