I suggest taking a look at your html sanitization settings make sure html isn't allowed in markdown settings.
Next step would be to move the other 2 topics from that category and check if you still see broken markup.
Re: Private Posts
Hi julian, curious for your reply below:
Fairly slim, as it is not in use so there is no incentive to keep it updated. A dedicated forum with stripped down permissions is doable already in core (though you don't get to allow users to reply to their topics only...)
I have tried some but didn't make it possible for user to see only their own posts in a forum. Otherwise I have to make a new plugin, but maybe I don't need to reinvent the wheel