5 level Queue Tree - No queuing on L4

I have recently added a 5th level to our Queue Tree, I noticed once I had done this that the stats for the 4th level in the queue tree were no longer updating, and traffic did not appear to be being queued at Level4 any more.

After disabling the 5th level, all goes back to normal.

Is this a bug, or a limitation ?
The Problem.png