Column net price is empty

[leo2024]

Dear expert:

         By t-code me31k, Create two service contracts document whose material group in contact item are same. the material group is 007 ( services ).  the different is services master data of service line in document，one service number of contract document is 100225, the other service number of document is 100227.  then, By t-code me51n, Create purchase requisition document with material group 007 ( services ) and service number 100225. Double click "assign source of supply" in source of supply tab, the system show source of list, but I find that the column net price in some document is empty. for example document 4600000106， the net price in some document is not empty. for example document 4600000107.

       Why the column net price in this two document show different behaviors. Is this standard behavior? I think this is problem, could you please check this problem in internet sap system. screen shots is as below.

Thank you