Tricky PIR Reduction Issue

Dear Experts,

Recently I found a tricky issue about PIR reduction, would you give hands and help?

Background: Strategy Group 40, default Setting with class type 101 & 50.

1) I have a DN with Qty 50. In the meanwhile the PIR is 30(Planned Qty) and the withdrawal qty of this item(in MD63 schedule lines) is 53.

2) Now I post DN GI (601), and then check MD63, PIR planned qty turns 0, and the withdrawal qty shows 83(=53+30, since 50>30), so far it's ok.

3) Then I reverse the DN GI via VL09, here comes the tricky problem: The PIR planned qty in MD63 bounce back to 50(in step one, the planned qty is just 30 before I post DN GI) but not back to 30, and the withdrawal qty is 33 but not 53.

It seems ridiculous, do you think so? Why SAP change the original PIR planned qty when reverse a DN? Imagine that users just create/post a wrong SO/DN and then reverse them, do I have to get additional PIR?

Sorry for that seems there's something wrong with my network therefore I can't upload the snapshot...

Hero Li