When you changed the period to period 12 for a transaction, all details of the transaction were changed to period 12, no matter what their dates were.

Inventory is a Control Account that should only be posted to by the Inventory module.

Could the problem be that your Credit Note in A/P was done as Debit Payables & Credit Inventory, but it should be Debit Payables & Credit Cost of Goods or another expense account, or possibly Receipt Clearing.