The value of WhenCreated is changed in Exchange Online
Symptoms
In Exchange Online, when you run the Get command to return objects such as user, mailbox, contact, or distribution group, you notice that the WhenCreated date is changed.
The following example returns a WhenCreated value about Mailbox01.
Get-Mailbox -Identity Mailbox01 | FL *WhenCreated
WhenCreated : 6/17/2018 5:16:56 AM
After you wait a while, you run this command again, and you notice that the value of WhenCreated is changed.
Get-Mailbox -Identity Mailbox01 | FL *WhenCreated
WhenCreated : 10/4/2019 6:23:56 AM
Cause
This behavior is by design. The WhenCreated date is not guaranteed to remain the same. One example of why it might change is after Tenant Relocation runs. This is because objects are re-created instead of moved during the process. Therefore, you should not rely on the WhenCreated value to do other operations that require that the value remains the same.