When writing in a oversized stack of book and quills, and signing the stack, the result item is a single signed book. This represents item loss of all other items in the stack except for one. I would expect either all book and quills to turn into written books, preserving the stack size, or only one book and quill is signed and the rest stay as book and quills.

Steps to reproduce

1. Obtain an oversized book and quill stack: /give @s minecraft:writable_book[max_stack_size=99] 99
2. Open the book and quill to sign it.
3. After signing, observe that the stack size is now 1 rather than 99 (or in addition to 98 book and quills)