When you want to change the user UPN, in certain conditions, this UPN change will not be synchronized to AAD (Office365/Intune/other).. why?
When you have federated domains for Office 365, or rather AAD in general and you want to switch your users from one domain to another, you will notice that that object will replicate anymore to AAD (and thus Office 365). I noticed this a long time ago, and it seems Microsoft now also posted this as a known KB a few weeks ago..