You can test it here https://www.svgheart.com/search-free-svg/
unmark all “free svg files” and mark category “svg and png sublimation” and nothing shows up.
When you mark any other category, all products assigned to this category will show up.
Is this bug or it must be like that?
Just to follow up, this is intended by default. The checkbox logic works as exclude anything unchecked by default.
It is possible to change it here: https://i.imgur.com/HdQFDAJ.png
It will very likely resolve the server load issue as well for using checkboxes, I forgot to mention this option in the other thread.