To reproduce:

1. Place a sign with text on it
2. Use a Glow Ink sack to make the default black text glow with a white boarder
3. Rotate your camera in 1st person such that you're viewing the sign from greater than 90° (so head on would be 0°)
4. Text on the sign turns fully white and is somewhat illegible
5. Alternative: Repeat steps 1 and 2 but look strait up from underneath the sign and rotate on a fixed axis. Text will flicker with white glow effect tearing

Seems to affect other colors other than Black, but effect isn't as noticeable/contrasted