It has been like this since for as long as I can remember. In the past we opened tickets for this and were told it is an Outlook issue. But it is not just Outlook (though I find it hard to believe Microsoft's own product has issues with its own protocol). Apple Mail has issues, the Android mail app, K9 Mail has been reported to have same issues from our customers.
Our customer who came from Office365 said this never ever happened across all his devices, first week with us and he is already having syncing issues.