This is strange and it is a theme hook or filter or whatever because with tweenty* themes is working as expected.
We have several categories, some created time ago, some recently created but with similar ids (15, 18, etc.)
In our category templates we call get_the_category() function to do some tasks. With older categories array returns expected data, with newer categories templates, array is empty.
As I said, when we use a default theme, everything is working.
Premium themes are not similar to default WordPress themes, that is a fact. So methods that work on default themes may not work on custom made themes. You can see how the theme gets the category here, that might be helpful to what you are trying to do
– https://www.screencast.com/t/fza9Yq55A8
– https://www.screencast.com/t/QGrqWEpw7GgO
