I'm having the same problem. I create .ics files daily and have never had a problem until I upgraded to Outlook 2016. I always create an html file. Open it in a browser (normally Firefox, but tried Chrome & IE), copy and paste it into the body of a calendar
file. I changed the format from Plain text to HTML Before pasting the copy into the body. I click, file, save as (oft) and when I double click to open it back up I get plain text with some formatting issues and my proofer, who did not upgrade sees the same
thing.
Also, I noticed that if i open others that were created before. They look bad to me, but are fine for others. It seems to open the old and new in Plain Text format and clicking HTML Format seems to do nothing even if I save it that way.
Mostly, I lose some of my line breaks, paragraphs, and the hyperlink formatting (as well as all other formatting)[View:/cfs-file/__key/communityserver-discussions-components-files/158/oft-issue.pdf:320:240].