Incorrect TQA and ATP quantity after delivery quantity change

Dear Gurus,

We have a problem in following scenario:

Stock - 20 PC, Sales Order – 15 PC

Step 1: Create delivery from SO

Step 2: Execute batch determination   (system confirm 15 PC – OK!)

Step 3: Change quantity in main item from 15 to 16 PC (Don’t delete subitem!)

Step 4: Again execute batch determination

Result: system confirm only 6 PC –  ATP quantity incorrect!

An analysis of the TQA report (/SAPAPO/AC06 - Temporary Quantity Assignments) shows that

After step 1 we have 3 records:

CAT        ReqQ            ConfQ

BM        -15                        0

BM          0                       -15

BR          15                      15

After step 3 we have 2 records:

CAT        ReqQ              ConfQ

BM           -1                      -1

BR            1                        1

Questions:

1) Is this the correct behavior of the system or program error?

2) How can we overcome this issue?

Thanks in advance!