Formatting in book and quill is different after signing

The bug

If you continue typing past the end of a line, (and the text wraps) the next line will still have the formatting from the previous line (this is normal), but when you sign the book, the formatting codes won't affect the new line anymore, and the text will look different.

Paste for example this text: §6TestTestTestTestTestTestTest

Code analysis

Code analysis by Marcono1234 can be found in this comment.