If the 50th page of a Book and Quill is deleted in a certain way, and a new page is inserted earlier in the book, the pages after the new page do not shift forward by one page as expected, but behave in an odd way and may delete the content of alternate pages by overwriting them with the content of other pages.

This has the potential to unintentionally permanently delete information important to people in their worlds.

Steps to Reproduce:

1. Get a book and quill with content on all 50 pages.
2. Edit page 50 to remove all text.
3. Press ESC to stop editing page 50.
4. Delete page 50 using the UI at the bottom of the page.
5. Go to any earlier page.
6. Add a new page using the UI at the bottom of the page.
7. Turn any number of pages forward in the book.
8. Exit the book by pressing ESC.
9. Open the book again.

Observed Results:

All pages after the new page you added and before (not including) the last page you viewed before closing the book will duplicate every odd-numbered page onto the even-numbered* pages, overwriting the original text on those pages. Every page after (and including) the last page you viewed will have not been moved forward by a page, and the empty page 50 will exist once again.

*Further explanation:

Typically if you add an odd-numbered page - e.g. page 21 - the page contents originally on page 21 will be on both page 21 and 22 (overwriting the empty page you added), and all the following duplicated pages (up to the last page you viewed before closing the book) will be duplicated onto their original pages and the next page in the same way, overwriting the page contents from the originally even-numbered pages.
If you added page 21 and last viewed pages 27+28, it would look like this: (starting from page 21) "21", "21", "23", "23", "25", "25", "27", "28"...

Whereas if you add an even-numbered page - e.g. page 22 - the empty page you added will remain and the duplicating won't begin until page 23 and 24, which will both have page 23's original contents. Which continues to the last page you viewed before closing the book.
If you added page 22 and last viewed pages 27+28, it would look like this: (starting from page 21) "21", "[blank page]", "23", "23", "25", "25", "27", "28"...

This seems to sometimes behave a little differently. When adding page 1 and last viewing pages 13+14 the pages were: "[blank page]", "2", "2", "4", "4", "6"... "10", "12", "13", "14"...

I think on some occasions the blank page 50 has been overwritten by the contents of page 49, so both pages are the same (which contradicts the way the last page you viewed and those after it seem to typically behave). This only occurred once for me (the original time the bug happened) and I haven't tried to reproduce it, nor do I know the cause.

Note that the page contents behave normally until the book is closed and reopened.

Expected Results:

The page contents after the new page should all be shifted forward by one page, none should be duplicated/deleted/overwritten, and the final blank page (50) should not reappear.

Notes:

Tested in 1.20.73 (only).

For testing I would recommend using creative mode and duplicating a book with 50 pages of content by putting it on a lecturn, and optionally using ctrl+pick block (middle click) to get a lecturn with the book's data.

Attached are images of the steps I followed in a new, superflat, creative mode world (though I don't think those factors matter, I've recreated it in a survival infinite world). I added a new page at page 1 and last viewed page 49-50 before closing. I have tested with other combinations not shown in the screenshots and they conform to the behaviour explained in the Observed Results section.

The bug first occurred today, in this current update (1.20.73) in a survival world, I don't know of it occurring or not before this.

This bug is important as others may lose text they've written in books unintentionally during normal gameplay, like I have.