Photo books keep pre-filling new pages with (random) images in Lr CC

I am now creating my 6th photo book with Lightroom Classic CC, and I have a continuing problem that I could not find any solution for (neither in the manual nor in the forum): when I want to create an (1) empty new book or add an (2) empty new page layout to a page, Lightroom automatically adds photos.

1. I disabled "automatic fill" in the Book preferences.
2. As a starting point, I created a new top-level collection set while in the Book module: magically, the film strip at the bottom showed a selection of random images, though – when looking at the collection set in the Library module – it was empty. (I also tried creating a collection set in the Library module, but when I switched to Book those ghost images would still be there.) The images came partly from previous books and partly from random folders in my library.
3. Then, I created a new book inside the collection set, leaving all options unchecked. My expectation was (empty collection, no selected images) that I would now see an empty book and that the book collection was also empty. But Lightroom automatically added some images from my previous books and also some random images to the book collection. (I switched to Library to make sure they were no ghost images this time.)

The second – even more annoying effect – is with adding new page layouts:

1. I created a new book in an existing collection set with other book projects, and I removed the unwanted random images.
2. Then, I started inserting page layouts (1, 2, 3 images per page etc.) into the blank pages... and some of those image positions in the layout were automatically prefilled with images from my previous book, and those images were now magically added to my new collection!

This is particularly annoying, as in a big project I now have to search the whole collection to remove images which are not supposed to be there.

Does anybody know what may cause these issues?

PS: Lightroom Classic CC 8.2 on MacOS 10.14