Inability to edit e-books in certain ways?

[Milk64]

I've been trying to edit some e-books I've downloaded on my 2018 Kindle Paperwhite, but it doesn't seem to be working.

Specifically, I've been trying to remove blank spaces, add Unicode ligatures, and add curly quotes to my e-books.

The first way I've done this is by using the convert function on Calibre, which lets you remove blank spaces and replace text with your own. Since I prefer AZW3, I just do AZW3 to AZW3. After doing this, I replace the old e-book files with these new ones. However, when I open the books on the Kindle, it's as if I'm still reading the old ones, and when I open the books in the Calibre editor, the new files have become like the old ones.

The same happens when I edit the books already on my Kindle directly in the Calibre editor. I use the search and replace function to add Unicode ligatures to the e-book, but they don't stick when I open them in Kindle, and the changes have reverted when I reopen them in Calibre editor. And I do save.

The weird thing is that both of these methods have worked for other e-books I've edited. I actually made two of my e-books myself using a program. These methods have worked on one of them, but not the other. I'm thinking it might be because I've edited too many? Could it be related to the Kindle's indexing function, or is it unrelated?

Also, for the e-book I made where the ligatures stay put, the ligatures are present in both the code and the preview. For the e-book where the changes don't stick after I save, the ligatures are still present in the preview when I open the book, but not in the editor. It's like it's being stored on my computer instead of in the e-book, which is strange.

The same thing happens with a different e-book when I try to add curly quotes to it, both using the convert's "smarten punctuation" feature, and manually.

This seems like a very specific problem. Has anyone else faced something like this?

I actually made this post on the Kindle subreddit, and someone told me to ask on this forum, so I made an account and copied the same post here and made a few edits.