It was happening to me as well. I assumed the cause was an update to the browser since it was happening at every board that has this format including MD. I went to "settings" then "General settings" and changed it from Enhanced Interface to Standard. That did the trick.
Message Editor Interface: Enhanced Interface - Full WYSIWYG Editing
Basic Editor - A simple text box
Standard Editor - Extra formatting controls
When posting messages to the forums or other members, there are three interface types available to you. The simplest of these is a simple text box, while the last is a fully-fledged WYSIWYG editor, which allows you to format your text as you want it and see the results immediately.
Depending upon the capabilities of your web browser, you may not be able to use all of these options. If you experience problems when posting messages, try switching to a different interface type.