I think I might understand better now, and actually there might be a possible solution as well (with a small custom code snippet).
I believe you want to show pages/posts that does not belong to any category, whenever there is nothing selected in the filter. Then whenever there is anything selected in the filter, then only show results that match those categories, but exclude everything that does not belong to any category – is this correct?
If yes, then this should be a possible solution:
1. I have tested the custom code below, with this advanced configuration (using checkbox filters), make sure that it configured like this: https://i.imgur.com/YApcZIB.png
2. Add this custom code to the functions.php in your theme/child theme directory (copy from line 3 only!). Before editing, please make sure to have a full site back-up just in case!
This will detect if no checkbox is selected whatsoever, and then it changes the parameters to allow the posts/pages without categories to display. Let me know if you need help with this custom code implementation. Please be very careful when editing your site files!Best,
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