So if I'm getting this right, it's okay when adding new contacts; but not when you're amending any??
Also, when you amend a contact on your phone, it saves on your phone okay, but not on your PC??
I've possibly got a way round this bug:
Select the contact you wish to amend.
Call the contact, but hang up straight away before it connects.
Delete the contact - hopefully this will delete it from your phone and PC - maybe wise to check and see.
Go into your call history and click on the last call you made. It should show up as just a number, as it'll no longer be in your contact list.
You can save it to your contacts, but you can edit it before you save it, so you can make whatever changes you need to and it should sync okay, as it's adding a new contact. The only issue I can see, is if it syncs okay when you delete the contact on your phone??