why Order line is not splitting to two lines when sales ord qty > available qty

Summary

Sales order qty is more than available qty,ideally it should get split ,but it is not

Content

We are in Rel 13 and update 19A,we have an Item A ,say qty is 20 but available qty is 10 ,so when we submit an order ,expectation is there should be 2 lines ,one line qty 10 should go to awaiting shipping after schedule and reservation and line 2 should be 10 may be waiting in scueduled as reservation is not available.

I had seen this happening but now it is not happening,why is that?

 

Can any expert comment on this properly.