Hybrid setup and only 15 out of 2500 users are getting mailbox provisioned

[JimmyStewart]

We do not want users to get their mailboxed provisioned in Exchange Online.  

In order to enable our E5 Security licenses, we need to give users an Exchange Online licenses.

We have about 2500 users... and generally, we do not have any issues... we give the Exchange Online license to a user and they do NOT get a mailbox provisioned because they already have it on premise.   This is expected behavior.  However, we have about 15 users who are getting provisioned in Exchange online and I cannot identify why.  They have an msExchMailboxGUID in their AD record and it syncs to Azure AD.  That should be the end of it... but alas, it is not.  

Has anyone had this issue and/or shed some light?  

Thought I'd ask the community before opening a ticket with Microsoft. 

Thanks

[#2 · Accepted solution]

Ah, I figured out how to remove the online mailboxes and we are now good... for anyone that comes across this... here is an article that outlines what to do. https://techcommunity.microsoft.com/t5/exchange-team-blog/permanently-clear-previous-mailbox-info/ba...

In gist, remove the exchange online license from the user.
run the Set-User [USER] -PermanentlyClearPreviousMailboxInfo powershell command
run a delta AD Sync to make sure Azure AD picks up the Mailbox GUID
Verify 365 is showing the users mailbox is on prem in the admin.microsoft.com/#/users console
re-apply the licenses and this time, a mailbox should not be created.