Our company sends HTML email invitations to meetings, which include the detailed agenda and a single simple CTA button to RSVP or join the meeting. We also include a calendar file that can be accepted onto the calendar.
The email itself all works just fine in the email clients. But, once someone accepts an event onto their Outlook 2013 or 2016 calendar, the email "disappears" from the email inbox. The content we sent in the invite now only appears in the description of the calendar event.

The calendar description is munging and/or stripping all the formatting from the HTML email body. The level of stripping goes from all fonts and buttons being lost - like being converted to RTF - to a full fallback to plain text. Using a new copy of Outlook 2016, I see it has tools for adding HTML formatting to event descriptions (although the formatting never seems to get saved?).

Two questions:
1) Has anyone found a reliable way to get an HTML email including a calendar file to survive once the event is accepted onto the 2013/2016 calendar?

2) Does anyone know a way to test this all reliably? I was hoping Litmus would help, but it's only showing me what happens when viewing the email in the Inbox, not what it looks like once it hits the calendar.