sub tag of the primary tag were gone

When you move a primary tag with sub tags to the Secondary tag, the http request show that the order is null which is correct, but when you move this tag to the primary Tags, the page show that the sub tag s were still inside the primary tag ,but the http request show the order had a id but no children tag,when you change the position of the sub tags, is turn normal。

http://i.imgur.com/z0Ne0mi.gifv