When editing books containing the formatting character '§' followed by a formatting code, only one of them is removed (see in the screenshot, where I pressed delete in the second line and it left he 'r' reset formatting code).
Use the following command and right click the placed sign to reproduce the issue:
/setblock ~ ~1 ~ minecraft:oak_sign{front_text:{messages:['{"text":"Click me","clickEvent":{"action":"run_command","value":"/give @s minecraft:writable_book[minecraft:writable_book_content={pages:[\\"\\u00a7nVery cool text\\n\\u00a7r \\u00a7\\u00a7\\u00a7\\u00a7 hi\\n\\u00a75more\\n\\u00a7etext\\"]}]"}}', '""', '""', '""']}}
In older version, it would be:
/setblock ~ ~ ~ oak_sign{Text1:"{\"text\":\"Click me\",\"clickEvent\":{\"action\":\"run_command\",\"value\":\"/give @p writable_book{pages:[\\\"\\u00a7nVery cool text\\n\\u00a7r \\u00a7\\u00a7\\u00a7\\u00a7 hi\\n\\u00a75more\\n\\u00a7etext\\\"]}\"}}"}
(This can also be reproduced without extra commands by having a book with the formatting character in a 1.13 world by copy-pasting text into it, and upgrading/opening that world with a 1.14+ client.)
What can be observed:
removing the '§' character doesn't remove the character setting the color afterward (and seemingly makes it pop into existence)
removing the color character doesn't remove the formatting character (and seemingly makes it pop into existence)
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Added an updated command, valid in 1.21.1
This command doesn't work anymore as of 24w36a so I cannot reproduce this.
Could you provide updated reproduction steps?