Unicode font is incorrectly highlighted on signs

Summary:

Unicode font is not highlighted correctly on signs.

Steps to reproduce:

1. Place signs.
2. Write in any language that is printed by Unicode (Russian, Ukrainian, Chinese).
3. Take Glow Ink Sac and highlight the text on signs.

Observed results:

The text will be highlighted with random stripes.

Expected results:

The text will be highlighted with a bold line.

Image: