Thank you for the details, I checked your configuration, it looks correct.
However I've looked this page in the back-end: https://www.holidayomat.com/en/holidays-australia/au-2017/
This is not categorized anywhere, there are no categories, see the back-end: https://i.imgur.com/Ts93Vrv.png
Pages doesn't have categories in WordPress as well, only posts and other custom post types as defined. The plugin displays results, because for pages the category filter doesn't apply. And even if it did, in this case it would return 0 results, because of the missing actual categories unfortunately.
The “holidays-australia/au-2017” is only part of the permalink structure, it does not define any connection between the elements unfortunately.Best,
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