Can't write "AHH! AHH!" on a sign

For some reason, when a sign contains the text "AHH!\nAHH!" (\n = line break), after accepting the text, the sign displays a sequence of `#` signs (like "#########").

The sign can contain "AHH!" alone, but it can't be written twice. Other numbers of H's and exclamation points also seem to trigger the issue. 

I was not able to determine a predictable combination of characters that leads to the display issue, other than this specific example.

 

Steps to reproduce:

1. Place a sign in the overworld.
2. Write the text "AHH!\nAHH!", where `\n` is a line break.
3. Accept the text.

Expected: Sign displays entered text.

Actual: Sign displays a sequence of `#`'s.