I tried different things, but can't reproduce it, neither on mediawiki.org nor locally.

Could it be that the steps to reproduce miss a step? After clicking "Create a book", there is no button "Add this category to your book". I have to click "Start book creator" first. Is this step missing for you?

I'm not sure if this is related, but I notice a confusing behavior on mediawiki.org. When I try to disable the book creator, it's like my wish is ignored. It appears again. Turns out this is a caching issue. When I shift+reload the page, the book creator finally disappears. Other actions behave similar, e.g. enabling the book creator in the first place. I can not reproduce this locally.