Obtained a librarian villager with mending and bought the book. Upon trying to apply it to an item at the anvil, it had turned into Unbreaking 3.

After a few more tries with new librarians and at different time intervals between obtaining the required librarian and buying the book, I came to the conclusion that the problem was still there and persistent. Upon updating the book by e.g. dropping and picking it back up or using an anvil, the mending book continued turning into Unbreaking 3.