The TargetAddress attribute does not get synced, it is on-premises only.
The reason for this is that there is no reason to actually sync it.
It is used for the following:
You have migrated a user from On-Prem to Office 365 or you have created a remote mailbox user on-premises that has a mailbox in Office 365.
The user is synced to Office 365 and the mailbox lives in Exchange Online.
Now, since Exchange is self aware, it will need an object locally to tell it where the user is to be found.
The Target Address says, for this remote user, send mail to this address, the mail then goes through the normal connector mailflow in your hybrid setup and lands in Office 365 where the mail is delivered as normal since it has been sent redirected to the address the user has in Office 365.
Usually for Target Address you would use the mail.onmicrosoft.com domain alias for the user, your on-prem connector points this domain to Office 365.
As soon as the mail lands in Office 365 the mail alias (proxyaddress) will tell Exchange Online where the mailbox is. This is why Office 365 doesn't need to know the Target Address attribute, all is handled by normal mailflow, it is only for On-Prem to know what to do since the mailbox is not in the local Exchange Organisation.