When an Item is renamed from an anvil it will return with the characters being #. It will display the new name in the anvil when changing the text but alters the name to # when you remove it from the anvil.
For example, putting in any item and trying to rename it, "Test" will return #### as the new item name after you remove it from the anvil.
I tried with multiple different items including name tags and even tried reloading the world.
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There are also reports already for book&quill.

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Not sure if this is related, but I just had a hunch for book and quils.
When signing a book it does not produce the name given nor does it show the author. It just returns Written Book.