wpForo gives priority to the topics which have been created directly under the parent forum. It's normal. We'll take this under consideration to change the logic, but this is not bug, wpForo is designed with this logic.
Agree with Sacri. It will be more useful to see last posts of subcategories on main page.
Yeah I can understand the logic, 'cause you have an option on the main forum where you can see 4/5 last topics. And it gives more visibility to the topic under the parent forum.
But that's quite sad to don't see the last message from the sub-category. But have an option to choose which one we want to put first could be great! 🙂
Thanks for your answer!