According to the original pull request for AO3-4923, the load_bookmarkable method in the bookmarks controller has an unused branch at elsif params[:chapter_id]: https://github.com/otwcode/otwarchive/blob/d27be43e444c414dc0d5620aff0df81f05c0ea7c/app/controllers/bookmarks_controller.rb#L27-L28
We'd like to remove this.
Please go to a single chapter of a multichapter work and make sure you can still create a bookmark. Then return to the chapter and make sure you can edit the bookmark.
It wouldn't hurt to try it from a chapter URL that is missing the work parameters, e.g. https://test.archiveofourown.org/chapters/####. (Note: This may or may not redirect to the full https://test.archiveofourown.org/works/####/chapters/### URL. The original pull request notes that it does redirect, but I am currently able to access that URL on staging and production without an issue.)