The change log of the purchase order shows the transaction ME23N.
Item Changes will be saved in ME23N, if this transaction was used to enter PO. Item Changes will be saved in ME22N only if you go directly to this transaction to perform the changes.
In the header modules, you obtain the SAP transaction category via the parameter I_TRTYP:
-> A Display
-> V Change
-> H Create
The same is valid for ME52N and ME53N (Purchase Requisitions).
Related notes / content:
491789 FAQ: Purchase order (ME21N, ME22N, ME23N)
212447 ME22N: Activity category