I set up a discussion board a while ago (mid 2015) on our O365 SharePoint to test styling. I cannot see anything different to the default set up except for a custom view. But, when I reply to a post on the discussion board, the HTML tags in the reply are visible unless I edit the source code (which is a pain in the bum and most users will not know or want to know how to do).
Here is a clipped screenshot showing you what I mean:
Does anyone know why it could be doing this?