Sync issues between Eight and your iPhone contacts can occur if your contacts are also syncing with your external mail accounts. This is most commonly an issue when contacts are syncing with Exchange/Hotmail.
If you are having issues, please try turning off contact sync with these additional accounts:
Go to ‘Accounts & Passwords’ in your iPhone’s settings.
Tap on your external accounts and turn off ‘Contacts’.
You can turn the contact syncing back on again after you have exported your Eight contacts, but please be aware that this may mean Eight cannot keep the contact information up to date.