After some testing, i was able to reproduce the issue as well. As this is really a severe bug, a SevA case was opened at my companies account.
You can read and get the full details on Tony’s post here.
Note: I’ve tested this in my companies environment with the same Outlook client against Exchange 2016 CU14 and couldn’t reproduce the issue.
Once we have the root cause for this issue, you will be informed. Until then I can only recommend to open a ticket with MS.