"Add to Calendar" functionality in the New Outlook

When a user receives an email with a Calendar meeting attached to it, they are able to click on the dropdown in the attachment and select "Add to Calendar".

This functionality is supposed to add the meeting to the calendar without opening the attachment/invite itself. However, when the user clicks on "Add to Calendar", the meeting is displayed temporarily on the Calendar and then disappears.

If another calendar meeting is being "Added to Calendar", it doesn't show up at all, even temporarily. Occasionally, the second meeting is added successfully, however, the first meeting then disappears.

This behaviour is reproducible using different, independent accounts and devices - personal devices, corporate devices, personal Live accounts, corp email accounts, etc.

The only thing that does work is to open the attachment and use "+Add to Calendar" feature from the invite.

Any ideas on what is causing this or how to address this?