Cannot select subcategory field in entity I have a Structure section fruit with a single entry type fruit. 

I also have a Category fruitType with 2 levels: 


Citrus


Orange
Lemon
Lime

Melons


Watermelon
Honeydew



The entry type fruit has a Category field for fruitType. 

If I select a level 2 category for the fruitType field, say Orange: 



Craft CMS dismisses the category selection pop-up and the parent category is shown instead, in this case Citrus. 



From the browser's network inspector, I noticed that a POST request to /admin/actions/elements/getCategoriesInputHtml with the following form data: 

categoryIds[]:893
locale:en_us
id:fields-fruitCategory
name:fields[fruitCategory]
limit:1
selectionLabel:Category


However, the response from the server is: 

{"html":"&lt;input type=\"hidden\" name=\"fields[fruitCategory]\" value=\"\"&gt;\n\n&lt;div id=\"fields-fruitCategory\" class=\"categoriesfield\"&gt;\n\t&lt;ul class=\"elements structure\"&gt;\n\t\t\t\t\t&lt;li id=\"fields-fruitCategory-category-887\"&gt;\n\t\t\t\t\t\t\t\t&lt;div class=\"row\" style=\"margin-left: -0px; padding-left: 0px;\"&gt;&lt;div class=\"element removable\" data-id=\"887\" data-locale=\"en_us\" data-status=\"enabled\" data-label=\"Faculty\" data-url=\"http:\/\/craft.local\/fruit\/citrus\" data-level=\"1\" data-editable&gt;&lt;input type=\"hidden\" name=\"fields[fruitCategory][]\" value=\"887\"&gt;&lt;a class=\"delete icon\" title=\"Remove\"&gt;&lt;\/a&gt; &lt;div class=\"label\"&gt;&lt;span class=\"status enabled\"&gt;&lt;\/span&gt;&lt;span class=\"title\"&gt;Citrus&lt;\/span&gt;&lt;\/div&gt;&lt;\/div&gt;&lt;\/div&gt;\n\n\t\t\t\t\t\t\t&lt;\/li&gt;\n\t\t\t&lt;\/ul&gt;\n\n\t&lt;div class=\"btn add icon dashed\"&gt;Category&lt;\/div&gt;\n&lt;\/div&gt;\n\n"}


Based on the response and what's rendered in the interface, Citrus is shown and not Orange, despite Orange being selected.
Unlike structured entries, when you relate a category to another element, its ancestors are also included in the relation. Due to the way Craft looks up these ancestors when relating the selected categories, to enforce this behaviour, the ancestors are added to the relation before the one you selected, and it looks like you're limiting that Categories field to 1, so only the parent is being added.

Try setting the limit on that Categories field to 2.