It is standard behavior.
Please check following two notes.
549583 - FAQ: Order confirmation status of third-party and individual purchase order -> Q4
550192 - FAQ: Changing third-party items and individual purchase order items -> Q3
(Confirming more quantity in PO is just like decrease the order quantity in SO. That's why I ask you to check Q3.)
The PO updates the SO with function module: SD_PURCHASE_CHANGE_ORDER.
If you don't want the PO to update SO, you may consider modifying the standard system.
Also, you may prefer the system updating the SO confirmed quantity with the GR quantity, instead of PO confirmed quantity. In this case, please check below two notes.
149421 - Individual purchase order: No SD update by goods receipt
650247 - Delivery group date not brought forward for third-party/individual purchase order
hope it helps.
At last, I would like to ask a question.
Why do you change the PO quantity to a higher value? What is the business purpose?
Best regards,
Hualin