I came across this issue recently when working on my ‘posts‘ page on this website.
Take a look at the breadcrumbs at the top of this page. You will see it says ‘Home > Guide > WordPress > Post Name‘. The problem I was experiencing is that it would only show ‘Home > Guide > Post Name‘ without including the subcategory of ‘WordPress’. This was also reflected in the URL and would only show it under ‘Guide’ without including the subcategory.
When editing the post, you need to only select the subcategory, not the parent category. In this case I would only select ‘WordPress’ and not ‘Guide’. So the solution is to only select the last child category.
Here’s a screenshot as an example of how your post category settings should be (although with ‘Android’ as the subcategory instead of ‘WordPress’):