Update: I’ve discovered that the text is deleted/made blank whenever the book is closed and it is reloaded in the player’s hand, much like when the player switches to a sword and the sword is loaded and raised up…when the book doesn’t flinch like this and is reloaded, the text stays, but when it flinches/loads it disappears, and the book flinches/loads every time it is signed, thus deleting all of the text and thus making it impossible to complete and sign book and quills with text inside them (the first page remains untouched by the deleting and blanking oddly enough, but the rest of the book’s text is deleted)
This is still a problem as of 1.21.11