I have had signs placed in 1.12.2 and earlier getting black spots. I haven't used any special characters on the signs apart from å and ö. I have noticed it multiple times.

Screenshot attached.

I reproduced it by spamming some text on a sign in 1.12.2 and opening the world in 1.13. It doesn't always work for me.

World downloads:

• 1.12.2
• 1.13