field from category not available via craft.categories my categories have a custom field called titleTag for SEO as well as a number of other fields. When I render them at their URI (/categories/cat1) I can output those fields easily with {{category.titleTag}} etc. However on the main category page I also have a filter button that loads another URI via jQuery.load() to filter the items based on category. So in the filter template I have this to start.

{% set filter = craft.request.getParam('filter') %}
{% set category = craft.categories.slug(filter) %}


I then want to output (amongst other things) the custom field values of the found category like so {{category.titleTag}} but it comes out null (the category found I am testing with has the field filled out.

Where am I going wrong?
Because your category variable is no object of type craft\elements\Category but an ElementQuery in order to fetch the category you need to use the one() function

{% set category = craft.categories.slug(filter).one() %}  
{% if category != null %}
    {{ category.titleTag }}
{% endif %}


Your {{category.titleTag}} printed the search criteria. Imagine you would like to set other conditions in your query: how is Craft supposed to know when your query ends?