OK, it looks like an unintentional UX implementation.
In desktop view, you only see the time of the last post for topics shown on the category page.
In desktop view, you only see the time of the last post for topics on the topic list page.
In mobile view, you only see the time of the last post for topics on the topic list page.
In mobile view, you only see the time of the first post for topics on the category page.
Which of these things is not like the other? 