Hi RorlTtime,
Do you mean you have already added the ProxyAddresses back manually?
How do you add it?
Based on my experience, we can use adsi edit to manage properties, please edit it and sync to the Office 365.
After that, please force DirSync to update the change, and then wait a little time.
For how to prevent this information from being removed, currently you can only use the local admin permission to protect it.
If I have misunderstood your requirement, please free feel let us know.
Thanks,
Jason Jiang