Hi Casper Wang,
Here WE06 in NOT triggered, instead, since it is an Account assigned purchase order ( Hope you have sales order no in acct assignment tab in PO), the Txn keys KBS & WRX are triggered.
KBS- acct assigned PO,
WRX-GR/IR clearing.
KBS takes the account modification key from the definition of Account assignment category ( T Code : OME9) as shown in your above screen shot, it has taken the Account modifier ZIC & have found a G/L account for this, in combination with Valuation Grouping code & Valuation class.
Check your entries in the system in Txn: OBYC for ZIC/Val Class/VG, you can understand the same.
Thanks,
Kalai.