Yes, it's an expected behavior. Cause these users created before DirSync cannot match the AD users actually, and this synchronization is a fake sync, O365 tags/adds number on their aliases to distinguish them, it treats them as total different accounts.
If you have a couple of users previously created on O365 before dirsync, you need to perform the DirSync soft-match, plea check this :support.microsoft.com/.../2641663
And there is only the first synchronization can be done with the soft match (SMTP matching), depends on the fact that you have run dirsync, you may need the hard-match: blogs.technet.com/.../how-to-do-hard-match-in-dirsync.aspx
After enabling dirsync, all synced users and matched users need to be managed from local AD, they are unable to be edited in O365, it's expected.
