XF 1.1 Subforums appear outside, same level of "father"

Just migrated, i'm reorganizing single forums before officially opening my forum.

I created an "Archive" forum where i'm putting into a lot of unused forum. In other words, those forums are child of Archive but in the main forum list i see those forums at the same level of Archive.

What could be the issue ?
Rob, i created a node (Forum type) as a root node. So this node doesn't belong to any category. In this case i see all subforum of this node at the same level of the node. I don't think this make sense.

In order to actually put (and view) the subforum into the created forum, you have to put this "container" forum under another node of category type.