3.3.0 VLC iOS added support for vorbis comment METADATA_BLOCK_PICTURE so covers show on opus chaptered audiobooks. Before 3.3.0 opus chapters worked and also worked until 3.3.4. So just installed 3.3.5 and something broke opus chapters. If you click chapters now nothing happens.
sidenote: I use freac to make opus audiobooks at 16kbps and encode to a single file. On android VLC displays chapters in a smaller font so easier to view the entire chapter names that are long. Would be nice to have that option on iOS. Even if you turn in landscape mode sometimes long chapter names don't entirely show. Some audiobook apps deal with this by allowing two lines per chapter.