Hello Chris,
I believe the issue may that the service bus connector used by password-write-back within Azure, has stopped responding.
The resolution would be to disable and re-enable the password-write-back functionality, and restart the Microsoft Azure AD Sync service, then finally check in the Event Logs for Event ID 31005 (password-write-back up and running).
Disable-OnlinePasswordWriteBack
Enable-OnlinePasswordWriteBack
Net Stop ADSync
Net Start ADSync
Hope this fixes your issue,
Rann Sanders, MCSE, MCSA: Messaging