Hi Darren,
After consulting the issue with my senior members, they give me some tips about the issue. To narrow down it, can you please perform the following test?
- Create a new user and user mailbox on the on-premise server. In AD, set the user’s password to never expire and un-tick the option “User must change password at next logon”.
- Sync the user to Office 365 by forcing a sync (http://technet.microsoft.com/en-us/library/jj151771.aspx#BKMK_SynchronizeDirectories).
- Migrate the user mailbox to Office 365, then check if the issue persists.