Chat filter only works when window is in focus

The chat filter seems to let censored words through for about a frame or so upon gaining focus.

Steps to reproduce:

1. Open chat & type censored word
2. Leave chat open and bring another window in front of Minecraft on the same monitor
3. Refocus Minecraft and the censored word appears for about a frame before being censored again

The best way to show it is with a short video.