How to understand :
1_ open the project available here :
http://addiktion.free.fr/Forums/Reaper/folder_issue.RPP
2_ hide tracks 3 and 4 (named : 3 - F1 Track2 FolderA and 4 - F1 FA Track1)
3_ try to set track 7 as a track 2's child
> If you drag and drop your track below track 2, you make it a child of track 3, which is not visible! I think it should not behave like that, as you cannot group to the folder you want, and you group your track to an invisible folder (if you have a lot, it can be a mess and you have to make everything visible to group normally).
4_ try to set track 7 as a track 1's child
> Of course, you can drag and drop track 7 below track 1, but if you want to make track 7 the last track of Folder 1 (speaking about tracks order), you cannot, because it will again group to track 3.
I hope it is clear :)